Fix bad merge of D16791 and D16767

Summary: A bad Auto-Merge caused log buffer is flushed twice. Remove the unintended one.

Test Plan: Should already be tested (the code looks the same as when I ran unit tests).

Reviewers: haobo, igor

Reviewed By: haobo

CC: ljin, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16821
DBStress cleanup

Summary:
*) fixed the comment
*) constant 1 was not casted to 64-bit, which (I think) might cause overflow if we shift it too much
*) default prefix size to be 7, like it was before

Test Plan: compiled

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16827
Revert "Fix bad merge of D16791 and D16767"

This reverts commit 839c8ecfcd486f0db82ecb755a137ad95909966f.
Correct version of D16821
Don't sync in stress test

Summary: Syncing in stress test makes it run much much much slower. It also doesn't add much value IMO.

Test Plan: no

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16839
A heuristic way to check if a memtable is full

Summary:
This is is based on https://reviews.facebook.net/D15027. It's not finished but I would like to give a prototype to avoid arena over-allocation while making better use of the already allocated memory blocks.

Instead of check approximate memtable size, we will take a deeper look at the arena, which incorporate essential idea that @sdong suggests: flush when arena has allocated its last and the last is "almost full"

Test Plan: N/A

Reviewers: haobo, sdong

Reviewed By: sdong

CC: leveldb, sdong

Differential Revision: https://reviews.facebook.net/D15051
DB stress with normal skip list

Summary:
Hash skip list has issues, causing db_stress to fail badly.

For now, switching back to skip_list by default before we figure out root cause.

Test Plan: db_stress is happy(ier)

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16845
fix VerifyDb in StressTest

Summary:
this should fix the hash_skip_list issue, but I still see seqno
assertion failure in the last run. Will continue investigating and
address that in a different diff

Test Plan: make whitebox_crash_test

Reviewers: igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16851
Revert "DB stress with normal skip list"

This reverts commit 86926d8c6ab00e2acd1e1a99590cc18a1a57a62f.
Fix extra compaction tasks scheduled after D16767 in some cases

Summary:
With D16767, there is a case compaction tasks are scheduled infinitely:
(1) no flush thread is configured and more than 1 compaction threads
(2) a flush is going on by one compaction hread
(3) the state of SST files is in the state that versions_->current()->NeedsCompaction() will generate a false positive (return true actually there is no work to be done)
In that case, a infinite loop will be formed.

This patch would fix it.

Test Plan: make all check

Reviewers: haobo, igor, ljin

Reviewed By: igor

CC: dhruba, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16863