- Engine and TLEngine assume "threadlocal" behavior on Pool; both use connect()

- Engine and TLEngine assume "threadlocal" behavior on Pool; both use connect()
for contextual connection, unique_connection() for non-contextual.
- Pool use_threadlocal defaults to True, can be set to false at create_engine()
level with pool_threadlocal=False
- made all logger statements in pool conditional based on a flag calcualted once.
- chagned WeakValueDictionary() used for "threadlocal" pool to be a regular dict
referencing weakref objects.  WVD had a lot of overhead, apparently.  *CAUTION* -
im pretty confident about this change, as the threadlocal dict gets explicitly managed
anyway, tests pass with PG etc., but keep a close eye on this one regardless.