Incremental CAgg Refresh Policy

[fabriziomello]

Nowadays a Continuous Aggregate refresh policy process everything only once independent of how large the refresh window is. For example if you have a hypertable with a huge amount of rows it can take a lot of time and requires a lot of resources in terms of CPU, Memory and I/O to refresh a CAgg, and all the aggregated data will be visible for the users only when the refresh policy complete it execution.

This PR add the capability of a CAgg refresh policy be executed incrementaly in "batches". Each "batch" is an individual transaction that will process a small fraction of the entire refresh window, and once the "batch" finishes the execution the data refreshed will already be visible for the users even before policy execution end.

To tweak and control the incremental refresh some new options was added to add_continuous_aggregate_policy API:

• buckets_per_batch: number of buckets to be refreshed by a "batch". To summarize this value is multiplied by the CAgg bucket width to determine the size of the batch range. Default value is 0 (zero) that means it will keep the current behavior of single batch execution. Values less than 0 (zero) are not allowed.
• max_batches_per_execution: maximum number of batches to be executed by a policy execution. This option is used to limit the number of batches processed by a single policy execution, so if some batches remain next time the policy run they will be processed. Default value is 10 (ten) that means that each job execution will process the maximum of ten batches. Values less than 0 (zero) are not allowed.

[codecov]

Codecov Report

Attention: Patch coverage is 81.72043% with 34 lines in your changes missing coverage. Please review.

Project coverage is 81.88%. Comparing base (59f50f2) to head (3ee29cd).
Report is 806 commits behind head on main.

Files with missing lines	Patch %	Lines
tsl/src/continuous_aggs/refresh.c	77.39%	9 Missing and 17 partials ⚠️
tsl/src/bgw_policy/continuous_aggregate_api.c	85.71%	2 Missing and 1 partial ⚠️
tsl/src/bgw_policy/job.c	89.65%	0 Missing and 3 partials ⚠️
src/ts_catalog/continuous_agg.c	80.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7790      +/-   ##
==========================================
+ Coverage   80.06%   81.88%   +1.81%     
==========================================
  Files         190      247      +57     
  Lines       37181    45627    +8446     
  Branches     9450    11418    +1968     
==========================================
+ Hits        29770    37361    +7591     
- Misses       2997     3767     +770     
- Partials     4414     4499      +85     

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[mkindahl]

Some minor comments. Since it is in draft, I will wait with approving until you have the final version.

[mkindahl]

A few questions regarding some parts of the code where I am not sure if it is correct or not.