Improve ACL SETUSER Thread Handling Alternate Approach

[kevinmichaelbowersox]

This pull requests fixes issue #988 . Please see the detailed discussion provided in the issue for a deeper discussion of the problem this PR addresses.

Quick Problem Overview

The ACL SETUSER command may produce inconsistent results when multiple clients/threads attempt to concurrently modify the same user. Please find test cases in this PR that demonstrate the concurrency issues this PR addresses.

Solution Overview

This PR addresses the issue by implementing the following changes:

• The ACL SETUSER implementation was modified to perform a thread-safe read, copy, replace sequence of the User. This avoids mangling of details about the User when under load and receiving simultaneous updates from multiple clients to update the same user.
• A UserHandle object was introduced that is used for all access to a User and used for replacing the User with a modified version of the User object. Types that rely upon User obtain the UserHandle and then call GetUser to retrieve the most recent version of the user including any modifications. UserHandle provides a SetUser method that can perform a thread safe replacement of the User referred to by the UserHandle.
• Updates to types that previously depended on User to now use UserHandle and UserHandle#GetUser().
• Added a try-catch block in ACL SETUSER implementation that handles concurrent attempts to create the same user, avoiding an exception that was previously thrown when the user already existed in the map.
• Updates to the AdminCommands to leverage the UserHandle for obtaining the User during access control decisions. See benchmarks below.

Testing and Verification

• Added unit tests that repro the situation prior to making changes and verified existence of the issue.
• Performed local testing manually to verify expected results used in test cases.
• Verified new and existing tests passed once fix was implemented.

Other
Additional discussion regarding this issue can be seen in this draft PR where another solution was evaluated and discarded: #989

[kevinmichaelbowersox]

Benchmark with Changes from SHA (977bcb4)

Method	Params	Mean	Error	StdDev	Allocated
Set	ACL	14.41 us	0.148 us	0.115 us	-
SetEx	ACL	20.03 us	0.064 us	0.060 us	-
SetNx	ACL	21.66 us	0.050 us	0.044 us	-
SetXx	ACL	23.39 us	0.064 us	0.057 us	-
GetFound	ACL	15.52 us	0.066 us	0.062 us	-
GetNotFound	ACL	10.66 us	0.022 us	0.021 us	-
Increment	ACL	22.12 us	0.049 us	0.043 us	-
Decrement	ACL	21.74 us	0.051 us	0.045 us	-
IncrementBy	ACL	26.26 us	0.123 us	0.116 us	-
DecrementBy	ACL	26.16 us	0.063 us	0.056 us	-
Set	AOF	21.32 us	0.096 us	0.085 us	-
SetEx	AOF	27.83 us	0.197 us	0.165 us	-
SetNx	AOF	28.04 us	0.225 us	0.211 us	-
SetXx	AOF	29.43 us	0.105 us	0.093 us	-
GetFound	AOF	15.84 us	0.028 us	0.025 us	-
GetNotFound	AOF	10.60 us	0.016 us	0.013 us	-
Increment	AOF	27.27 us	0.055 us	0.052 us	-
Decrement	AOF	28.56 us	0.071 us	0.063 us	-
IncrementBy	AOF	32.46 us	0.163 us	0.136 us	-
DecrementBy	AOF	34.28 us	0.182 us	0.170 us	-
Set	None	14.33 us	0.024 us	0.023 us	-
SetEx	None	19.56 us	0.049 us	0.046 us	-
SetNx	None	20.45 us	0.035 us	0.031 us	-
SetXx	None	23.25 us	0.066 us	0.055 us	-
GetFound	None	15.21 us	0.157 us	0.139 us	-
GetNotFound	None	11.03 us	0.020 us	0.019 us	-
Increment	None	22.11 us	0.084 us	0.074 us	-
Decrement	None	22.69 us	0.149 us	0.140 us	-
IncrementBy	None	27.65 us	0.133 us	0.118 us	-
DecrementBy	None	27.63 us	0.192 us	0.180 us	-

[kevinmichaelbowersox]

Baseline Benchmark from Main from SHA (61567d5)

Method	Params	Mean	Error	StdDev	Allocated
Set	ACL	14.10 us	0.043 us	0.034 us	-
SetEx	ACL	20.34 us	0.063 us	0.052 us	-
SetNx	ACL	20.31 us	0.064 us	0.057 us	-
SetXx	ACL	23.62 us	0.029 us	0.024 us	-
GetFound	ACL	15.57 us	0.040 us	0.036 us	-
GetNotFound	ACL	10.79 us	0.047 us	0.039 us	-
Increment	ACL	21.36 us	0.042 us	0.037 us	-
Decrement	ACL	21.76 us	0.062 us	0.052 us	-
IncrementBy	ACL	26.17 us	0.099 us	0.083 us	-
DecrementBy	ACL	25.66 us	0.045 us	0.035 us	-
Set	AOF	21.20 us	0.062 us	0.058 us	-
SetEx	AOF	27.06 us	0.091 us	0.081 us	-
SetNx	AOF	28.04 us	0.119 us	0.106 us	-
SetXx	AOF	26.79 us	0.060 us	0.056 us	-
GetFound	AOF	15.46 us	0.053 us	0.047 us	-
GetNotFound	AOF	11.10 us	0.058 us	0.051 us	-
Increment	AOF	26.50 us	0.065 us	0.061 us	-
Decrement	AOF	26.89 us	0.072 us	0.060 us	-
IncrementBy	AOF	31.63 us	0.138 us	0.129 us	-
DecrementBy	AOF	32.17 us	0.136 us	0.127 us	-
Set	None	14.61 us	0.035 us	0.033 us	-
SetEx	None	19.76 us	0.053 us	0.044 us	-
SetNx	None	20.95 us	0.027 us	0.024 us	-
SetXx	None	22.19 us	0.064 us	0.060 us	-
GetFound	None	15.60 us	0.013 us	0.011 us	-
GetNotFound	None	12.22 us	0.040 us	0.036 us	-
Increment	None	22.16 us	0.051 us	0.043 us	-
Decrement	None	21.95 us	0.016 us	0.013 us	-
IncrementBy	None	26.43 us	0.060 us	0.050 us	-
DecrementBy	None	26.95 us	0.074 us	0.065 us	-