Rename receive_single_snapshot/4 -> receive_snapshots/4

The name "receive_single_snapshot" wasn't accurate as one request
may contain multiple snapshots.

This is pure renaming without a functional change.

Change-Id: Ibe3e2aa194956bccb4eadfb57e42b554cfd468e5
Reviewed-on: http://review.couchbase.org/33744
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>