PGLog: add a config to disable PGLog::check()

This is a debug check which may be causing excessive
cpu usage.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 00080d785f6695b800f71317a3048a21064e61cb)