Support multiple env instance

[lhsoft]

Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://groups.google.com/forum/#!forum/rocksdb or https://www.facebook.com/groups/rocksdb.dev

We have multiple disk on server and several rocksdb instances share one disk. It better to have one env per disk for isolation. Currently rocksdb support global env by Env::Default() and user can't create env because env constructor is private.

It better to add Env::NewEnv so user can create new default env.

Expected behavior

user can create multiple env instance by Env::NewEnv()

Actual behavior

currently only support env::Default()

Steps to reproduce the behavior

[pdillinger]

for isolation

What sort of isolation?