Performance enhancements on writes
Performance enhancements on writes

The focus of these changes is to avoid unnecessary SELECT and JOIN
when doing INSERT or UPDATE. Finding these things is a bit challenging,
it invovles a combination of granular profiling of code with time.clock()
and analysis of the SQL generated by sqlalchemy.

tiddlyweb/tiddlywebplugins.sqlalchemy,tiddlyweb/tiddlywebplugins.sqlalchemy