worker: process all bucket instance log entries at once

Currently if there are more than max_entries in a single bucket
instance's log, only max_entries of those will be processed, and the
bucket instance will not be examined again until it is modified again.

To keep it simple, get the entire log of entries to be updated and
process them all at once. This means one busy shard may block others
from syncing, but multiple instances of radosgw-agent can be run to
circumvent that issue. With only one instance, users can be sure
everything is synced when an incremental sync completes with no
errors.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
worker: convert list to tuple so addition to set works

Without this, an error like: TypeError: unhashable type: 'list' would
occur when metadata was retried.

Fixes: #7526
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>