thrift: Implement batch_mutate on top of storage_proxy
thrift: Implement batch_mutate on top of storage_proxy

So that the specified consistency level can be respected.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

raphaelsc/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla