Compare mappers more accurately in Load._chop_path

Compare mappers more accurately in Load._chop_path

Fixed bug in cache key generation for baked queries which could cause a
too-short cache key to be generated for the case of eager loads across
subclasses.  This could in turn cause the eagerload query to be cached in
place of a non-eagerload query, or vice versa, for a polymorhic "selectin"
load, or possibly for lazy loads or selectin loads as well.

Change-Id: I2a69349d3e38814e2c7e6012fc04fbc0e47658a4
Fixes: #4287