database: apply streaming mutations with streaming priority
database: apply streaming mutations with streaming priority

We are flushing the streaming memtables with streaming priority, but
applying the mutations themselves is still done with normal priorities.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla