pgsql: Slow down driver by default, async parsing only on request.
pgsql: #require Thread.Thread.
pgsql: #require required.
Fix code to match docs.
Fix docs.
Doc fixes.
pgsql: Run callbacks in a separate thread to keep session afloat.
Avoid ?-> operator for now.
pgsql: Reduce the number of dynamically created threads.

This patch pushes most of the dynamically created threads into the
running local_backend.  It specifically does not push the gotdatarowdesc
one to avoid deadlocks.
Configure: Improved handling of empty ABI directories.
Add documentation about the custom backend.