Fixed multitenant implementation so that Model instances are always tied
to their corresponding collection (i.e. bypass dynamic tenant resolution
for an individual Model object once the Model is actually loaded).

This is causing a perf issue right now because it is re-creating a new
MultitenantDatastore object each time a Model needs a Datastore, which
is inefficient (and causing the LifeCycleBenchmark test to fail due to
perf overhead).