crimson/osd/../client_request: add note to can_serve_replica_read

crimson/osd/../client_request: add note to can_serve_replica_read

can_serve_replica_read uses PGLog::has_write_since, when checking for
writes we actually check if any pglog entry belongs to the head object.

The only two pglog entries that are of a clone object are:
1) At creation (pg_log_entry_t::CLONE)
2) At trimming (See remove_or_update)

In both cases, the there would be another pg log entry of the head.

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Add assertion in prepare_head_update to assert that the above is true.
The obc passed to prepare_head_update (by OpsExecuter) could also be
a clone object (after being resolved). However, write operations should
only occur to head - so let's verify that.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>