CCBC-183 Improve lcb_get_replica()

Now it is possible choose between three strategies:

1) LCB_REPLICA_FIRST: previously accessible and default now, when the
   caller will get first successful replica or error

2) LCB_REPLICA_ALL: Ask all replicas to send keys

3) LCB_REPLICA_SELECT: select particular replica by the index (from 1
   to 3 inclusive). This one is faster, but can generate false
   negatives

Change-Id: Ib50a578d6ac99a0e1cb05c6a6a82b7c0c13082a5
Reviewed-on: http://review.couchbase.org/27308
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>