Merge existing query params in baked lazy load

Fixed a long-standing regression that occurred in version
1.0, which prevented the use of a custom :class:`.MapperOption`
that alters the _params of a :class:`.Query` object for a
lazy load, since the lazy loader itself would overwrite those
parameters.   This applies to the "temporal range" example
on the wiki.  Note however that the
:meth:`.Query.populate_existing` method is now required in
order to rewrite the mapper options associated with an object
already loaded in the identity map.  Also, a custom defined
:class:`.MapperOption` will now cause lazy loaders related to
the target object to use a non-baked query by default unless
the :meth:`.MapperOption._generate_cache_key` method is implemented.

Fixed bug where the new :meth:`.baked.Result.with_post_criteria`
method would not interact with a subquery-eager loader correctly,
in that the "post criteria" would not be applied to embedded
subquery eager loaders.   This is related to :ticket:`4128` in that
the post criteria feature is now used by the lazy loader.

Change-Id: I899808734458e25a023142c2c5bb37cbed869479
Fixes: #4128

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