Merge existing query params in baked lazy load

Merge existing query params in baked lazy load

Corrected backport, was supposed to be in 1.2.5 however
never got backported.

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.

As part of this change, 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.
In particular, this repairs one regression which occured when
using the dogpile.cache "advanced" example, which was not
returning cached results and instead emitting SQL due to an
incompatibility with the baked query loader; with the change,
the ``RelationshipCache`` option included for many releases
in the dogpile example will disable the "baked" query altogether.
Note that the dogpile example is also modernized to avoid both
of these issues as part of issue :ticket:`4258`.

This is a cherry-pick / squash from:
4a31c30fa5ebd6af0e72937b21b2e5ee79e12631
2e46f73f35b9036287f5f567c09a8cb786fe5fd3
b9f428a589a1718efa20e5555be45ae3f767e89e

Change-Id: I899808734458e25a023142c2c5bb37cbed869479
Fixes: #4128