Summary:
Introduced a new category in the enum InfoLogLevel in env.h.
Modifed Log() in env.cc to use the Header()
when the InfoLogLevel == HEADER_LEVEL.
Updated tests in auto_roll_logger_test to ensure
the header is handled properly in these cases.
Test Plan: Augment existing tests in auto_roll_logger_test
Reviewers: igor, sdong, yhchiang
Reviewed By: yhchiang
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D41067
35cd75c379