Summary:
This enables configurable Envs without recompiling. For example, my
next diff will make env_test test an Env created by NewEnvFromUri(). Then,
users can determine which Env is tested simply by providing the URI for
NewEnvFromUri() (e.g., through a CLI argument or environment variable).
The registration process allows us to register any Env that is linked with the
RocksDB library, so we can register our internal Envs as well.
The registration code is inspired by our internal InitRegistry.
Test Plan: new unit test
Reviewers: IslamAbdelRahman, lightmark, ldemailly, sdong
Reviewed By: sdong
Subscribers: leveldb, dhruba, andrewkr
Differential Revision: https://reviews.facebook.net/D58449
