Config, Transaction, and Opening Revamp

New Features

• The Config::open method has been added to give
  Config a similar feel to std's fs::OpenOptions.
  The Config::build and Db::start methods are
  now deprecated in favor of calling Config::open
  directly
• A checksum method has been added to Tree and Db
  for use in verifying backups and migrations.
• Transactions may now involve up to 69 different
  tables. Nice.
• The TransactionError::Abort variant has had
  a generic member added that can be returned
  as a way to return information from a
  manually-aborted transaction. An abort helper
  function has been added to reduce the boiler-
  plate required to return aborted results.

Breaking Changes

• The ConfigBuilder structure has been removed
  in favor of a simplified Config structure
  with the same functionality.
• The way that sled versions are detected at
  initialization time is now independent of serde.
• The cas method is deprecated in favor of the new
  compare_and_swap method which now returns the
  proposed value that failed to be applied.
• Tree nodes now have constant prefix encoding
  lengths.
• The io_buf_size configurable renamed to
  segment_size.
• The io_buf_size configurable method has been
  removed from ConfigBuilder. This can be manually
  set by setting the attribute directly on the
  ConfigBuilder, but this is discouraged.
  Additionally, this must now be a power of 2.
• The page_consolidation_threshold method has been
  removed from ConfigBuilder, and this is now
  a constant of 10.