Summary:
In 3.10 the C++ code was extended with a MemEnv implementation. This
is now also available in RocksJava.

Changes:
- Extraced abstract super class Env
- Introduced RocksMemEnv
- Remove unnecessary disposeInternal method. The disposal of the default environment is managed by C++ so there needs to be no disposeInternal method in Java.
- Introduced a RocksMemEnvTest, which is aligned with the C++ equivalent.

Test Plan:
make rocksdbjava
make jtest

Reviewers: adamretter, yhchiang, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D35619