crimson/osd/osd_operations/client_request: merge recover_missing,

crimson/osd/osd_operations/client_request: merge recover_missing,
recover_missing_lock_obc and recover_missing_snaps into a single
OrderedConcurrentPhase

The reasons for this change are as follows:

1. the pipeline phase in which the obc loading is done has to be entered
"synchronouly", however, the leaving of an OrderedConcurrentPhase can't
be so, which means we can't enter_sync an OrderedConcurrentPhase from
an earlier OrderedConcurrentPhase directly;
2. on the other hand, the whole missing object/snaps recovery procedure
can and should be a OrderedConcurrentPhase.

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>