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>
PG: remove old log when we upgrade log version

Otherwise the log_oid will be non-empty and the next
boot will cause us to try to upgrade again.

Fixes: #6057
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Merge remote-tracking branch 'gh/next'