PGLog: move the log size check after the early return

There really are stl implementations (like the one on my ubuntu 12.04
machine) which have a list::size() which is linear in the size of the
list.  That assert, therefore, is quite expensive!

Fixes: #6040
Backport: Dumpling
Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: don't maintain log_keys_debug if the config is disabled

Fixes: #6040
Backport: Dumpling
Signed-off-by: Samuel Just <sam.just@inktank.com>
PGLog: maintain writeout_from and trimmed

This way, we can avoid omap_rmkeyrange in the common append
and trim cases.

Fixes: #6040
Backport: Dumpling
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #550 from ceph/wip-6040

Wip 6040

Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Loic Dachary <loic@dachary.com>