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