Speed up creating RLMRealm instances on background threads

Cuts the init time when there's a cached RLMRealm for a different thread from 3ms to 1.2ms.

Most of the remaining runtime is spent on validating that the table schema matches the object schema, which can probably be skipped, but not touching that file to avoid conflicts with other stuff in progress.

@alazier 
Some of the migrations tests fail without it due to the test code mucking about with the schema.