[CF] More tests

Summary: New unit tests for column families

Test Plan: this is a test

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16359
[CF] Make LogDeletionTest less flakey

Summary: Retry GetSortedWalFiles() and also wait 20ms before counting number of log files. WaitForFlush() doesn't necessarily wait for logs to be deleted, since logs are deleted outside of the mutex.

Test Plan: column_family_test

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16371
[CF] DifferentMergeOperators and DifferentCompactionStyles tests

Summary:
Two new column family tests:
* DifferentMergeOperators -- three column families, one without merge operator, one with add operator and one with append operator. verify that operations work as expected.
* DifferentCompactionStyles -- three column families, two with level compactions and one with universal compaction. trigger the compactions and verify they work as expected.

Test Plan: nope

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16377
[CF] Test fixes and speedup
[CF] WaitForFlush() instead of sleeping

Summary: If we sleep for 300ms the test fails in valgrind because it takes more than 300ms to flush. This way we WaitForFlush() when we're expecting flush, but still sleep and check if the flush happens even though it's not supposed to.

Test Plan: notest

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16401