Memoize AliasedClass, ClauseAdapter objects in joined eager load

Memoize AliasedClass, ClauseAdapter objects in joined eager load

Addressed some long unattended performance concerns within the
joined eager loader query construction system.  The use of ad-hoc
:class:`.AliasedClass` objects per query, which produces lots of column
lookup overhead each time, has been replaced with a cached approach
that makes use of a small pool of :class:`.AliasedClass` objects
that are reused between invocations of joined eager loading.
Callcount reduction of SQL query generation for worst-case joined
loader scenarios (lots of joins, lots of columns) is reduced by
approximately 270%.

Change-Id: Ie43beecf37945b2bb7fff0aaa597a597293daa18
Fixes: #3915