Add universal compaction to db_stress nightly build

Add universal compaction to db_stress nightly build

Summary:
Most code change in this diff is code cleanup/rewrite. The logic changes include:

(1) add universal compaction to db_crashtest2.py
(2) randomly set --test_batches_snapshots to be 0 or 1 in db_crashtest2.py. Old codes always use 1.
(3) use different tmp directory as db directory in different runs. I saw some intermittent errors in my local tests. Use of different tmp directory seems to be able to solve the issue.

Test Plan: Have run "make crashtest" for multiple times. Also run "make all check"

Reviewers: emayanke, dhruba, haobo

Reviewed By: emayanke

Differential Revision: https://reviews.facebook.net/D12369