objecter: resend unfinished lingers when osdmap is no longer paused

Plain Ops that haven't finished yet need to be resent if the osdmap
transitions from full or paused to unpaused.  If these Ops are
triggered by LingerOps, they will be cancelled instead (since
should_resend = false), but the LingerOps that triggered them will not
be resent.

Fix this by checking the registered flag for all linger ops, and
resending any of them that aren't paused anymore.

Fixes: #6070
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage.weil@inktank.com>
(cherry picked from commit 38a0ca66a79af4b541e6322467ae3a8a4483cc72)