- Pool classes will reuse the same "pool_logging_name" setting

- Pool classes will reuse the same "pool_logging_name" setting
after a dispose() occurs.

- Engine gains an "execution_options" argument and
update_execution_options() method, which will apply to
all connections generated by this engine.

- Added more aggressive caching to the mapper's usage of
UPDATE, INSERT, and DELETE expressions.  Assuming the
statement has no per-object SQL expressions attached,
the expression objects are cached by the mapper after
the first create, and their compiled form is stored
persistently in a cache dictionary for the duration of
the related Engine.

- change #3 required change #1 so that we could test
a set of mappers operating over the course of many engines without
memory usage increase.