Reordered methods in database wrappers.

* Grouped related methods together -- with banner comments :/
* Described which methods are intended to be implemented in backends.
* Added docstrings.
* Used the same order in all wrappers.
Fixed #18130 -- Made the isolation level configurable on PostgreSQL.

Thanks limscoder for the report and niwi for the draft patch.
Fixed #19962 - Added a note about SESSION_EXPIRE_AT_BROWSER_CLOSE and browsers that persist sessions.

Thanks David Sanders.
Fixed #19917 -- Added microseconds in default TIME_INPUT_FORMATS

Thanks minddust for the report.
Fixed #16888 -- Added microseconds in localized input formats