new hooks for bulk handling

When using dns-01 challenge and several certificates in a single DNS zone are to be created/renewed there are more serial bumps involved than theoretically needed. Consider certs for A.example.org and B.example.org are to be created/renewed. Then
the `deploy_challenge` handler is invoked twice -- once for each domain. In the first invocation it's not known to the hook if another one follows, so the DNS update for the first domain needs to bump the zone's serial and trigger a notify to the slaves. This is also an issue if the serial is based on the current time (seconds since epoch is a usual scheme used here) and the two updates are triggered in the same second. Also the first invocation has to block until all secondaries are updated, just to wait a second time in the second invocation.

However in a `bulk_deploy_challenge` hook all challenges could be passed together and then only a single serial bump + notify (per domain) + wait would be needed.

Symmetrically for `clean_challenge`.