add psql FOR UPDATE OF functionality
added ORM support
added LockmodeArgs
Merge branch 'for_update_of' of github.com:mlassnig/sqlalchemy into for_update_of
- work in progress, will squash
- fix up rendering of "of"
- move out tests, dialect specific out of compiler, compiler tests use new API,
legacy API tests in test_selecatble
- add support for adaptation of ForUpdateArg, alias support in compilers
- add support for specifying tables or entities for "of"
- implement Query with_for_update()
- rework docs and tests
- repair the "lockmode" functionality of load_on_ident().  slightly problematic
here is that "lockmode" is also public in Session.refresh().
- changelog, migration