crimson/osd: simplify cloning

[rzarzynski]

1. Drop head_os and any other thing that could be a counterpart of the classical old obs structure.
2. The cloning operation, now executed at the very beginning of OpsExecuter's life cycle, produces new_snapset that is applied to obc after executing all other operations. (execute+commit).
3. It also creates a pg_log_entry which is appended at the end of log_entires at the flush (preserved the orignal logic, havent't experimented with it yet).
4. TestLibRBD.TestIOToSnapshot is green without any reordering within txn (tested with other recent fixes for cloning).

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