Allow user to specify log level for info_log

Summary:
Currently, there is no easy way for user to change log level of info log. Add a parameter in options to specify that.
Also make the default level to INFO level. Removing the [INFO] tag if it is INFO level as I don't want to cause performance regression. (add [LOG] means another mem-copy and string formatting).

Test Plan:
make all check
manual check the levels work as expected.

Reviewers: dhruba, yhchiang

Reviewed By: yhchiang

CC: dhruba, igor, i.am.jin.lei, ljin, haobo, leveldb

Differential Revision: https://reviews.facebook.net/D16563
Buffer info logs when picking compactions and write them out after releasing the mutex

Summary: Now while the background thread is picking compactions, it writes out multiple info_logs, especially for universal compaction, which introduces a chance of waiting log writing in mutex, which is bad. To remove this risk, write all those info logs to a buffer and flush it after releasing the mutex.

Test Plan:
make all check
check the log lines while running some tests that trigger compactions.

Reviewers: haobo, igor, dhruba

Reviewed By: dhruba

CC: i.am.jin.lei, dhruba, yhchiang, leveldb, nkg-

Differential Revision: https://reviews.facebook.net/D16515
Make sure GetUniqueID releated tests run on "regular" storage

Summary:
With the use of tmpfs or ramfs, unit tests related to GetUniqueID()
failed because of the failure from ioctl, which doesn't work with these
fancy file systems at all.

I fixed this issue and make sure all related tests run on the "regular"
storage (disk or flash).

Test Plan: TEST_TMPDIR=/dev/shm make check -j32

Reviewers: igor, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16593
Merge branch 'master' into columnfamilies

Conflicts:
	db/compaction_picker.cc
	db/compaction_picker.h
	db/db_impl.cc
	db/version_set.cc
	db/version_set.h
	include/rocksdb/options.h
	util/options.cc