Summary:
SnapshotConcurrentAccessTest sometimes times out when running on the test infra. This patch splits the test into smaller sub-tests to avoid the timeout. It also benefits from lower run-time of each sub-test and increases the coverage of the test. The overall run-time of each final sub-test is at most half of the original test so we should no longer see a timeout.
Closes https://github.com/facebook/rocksdb/pull/3435

Differential Revision: D6839427

Pulled By: maysamyabandeh

fbshipit-source-id: d53fdb157109e2438ca7fe447d0cf4b71f304bd8