rgw: fix replica log indexing

Fixes: #8251

Previously we were indexing the replica log only by bucket name, even
though we were provided with the bucket instance id. Fix that, and also
add the option to be able to revert to the old behavior. For
radosgw-admin we can do it if --replicalog-index-by-instance=false is
provided. For the replica log REST api we now have the index-by-instance
http param (defaults to true).

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
utime: extend utime parsing

Since we sometimes encode it as <sec>.<usec>, also try to parse it using
this format.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>