URI-based Env selection for db_bench

URI-based Env selection for db_bench

Summary:
Added an option, --env_uri. When provided, it is used as an argument to
NewEnvFromUri(), which instantiates an Env based on it.

Test Plan:
built a simple binary that registers ChrootEnv for prefix "/", then
ran:

  $ ./tmp --env_uri /tmp/ --db /abcde

/tmp/ is the chroot directory and /abcde is the db_name. Then I verified
db_bench uses /tmp/abcde

Reviewers: sdong, kradhakrishnan, lightmark

Reviewed By: lightmark

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D59325