Summary:
Define INFO0 through INFO9 levels for users that want to have finer-grained
control over info log messages.

This also renumbers the DBG log levels slightly so that there are exactly 1000
levels between each major log level step.  This renumbering change should not
affect any code behavior, since these level values are primarily used in memory
and aren't saved persistently anywhere.

Reviewed By: yfeldblum

Differential Revision: D6690466

fbshipit-source-id: acd499cff6830a1d1b579f295bd4b2bac93b5ada