deprecated "selectalias" argument on eager loader, do use_alias=True

deprecated "selectalias" argument on eager loader, do use_alias=True
"eager alias" flag will propigate to child eager loaders so the full query comes
out OK.  mappers/properties have overhauled "copy" methodology.  mappers
are no longer "singleton" and no longer have elaborate "hash_key" methods - there
is a primary mapper associated with a class which is done via direct dictionary
relationship, and the options() method on mapper does its own lighter-weight caching
of created mappers.  the unitofwork does extra work with the mappers it receives
to insure its dealing with "the primary" mapper, so that properties can be more liberal
about which mapper they reference (i.e. not the primary one).
options() works better but still could use a looking-at to see that
its not wasteful.  simplified mapper() method in __init__.