Add "empty in" strategies; default to "static"

Add "empty in" strategies; default to "static"

The longstanding behavior of the :meth:`.Operators.in_` and
:meth:`.Operators.not_in_` operators emitting a warning when
the right-hand condition is an empty sequence has been revised;
a new flag :paramref:`.create_engine.empty_in_strategy` allows an
empty "IN" expression to generate a simple boolean expression, or
to invoke the previous behavior of dis-equating the expression to
itself, with or without a warning.  The default behavior is now
to emit the simple boolean expression, allowing an empty IN to
be evaulated without any performance penalty.

Change-Id: I65cc37f2d7cf65a59bf217136c42fee446929352
Fixes: #3907