osd: close split vs query race in consume_map

osd: close split vs query race in consume_map

Consider the race:

- shard 0 consumes epoch E
- shard 1 consumes epoch E
  - shard 1 pg P will split to C
- shard 0 processes query on C, returns DNE
- shard 0 primes slot C

Close race by priming split children before consuming map into each
OSDShard.  That way the query will either (1) arrive before E and before
slot C is primed and wait for E, or find the slot present with
waiting_for_split true.

Signed-off-by: Sage Weil <sage@redhat.com>