Replace unused postgresql jar with manual site copy

Once the executor is built, the resulting postgresql jar file should be
manually copied over to the gerrit site /lib folder. Add this to the
plugin documentation. Make the latter's executor-related section usable
and cleaner.

Before this change, the plugin-provided postgresql classes were not
loaded upon gerrit core dependencies that transitively depended on them.

Relational database support in Gerrit already expects driver files to be
put under site's lib folder, for account_patch_reviews to duly function.
Include that aspect in the amended plugin build documentation.

Bug: Issue 12810
Change-Id: I5485068c55c34cc8193ee784abaf903a6b60ce13
2 files changed