alternator: make "executor" a peering_sharded_service
alternator: make "executor" a peering_sharded_service

Alternator uses a sharded<executor> for handling execution of Alternator
requests on different shards. In this patch we make executor a subclass of
peering_sharded_service, to allow one of these executors to run an exector
method on a different shard: Any one of the shard-local executor instances
can call container() to get the full sharded<executor>.

We will need this capability later, when we need to bounce requests between
shards because of requirements of the storage_proxy::cas (LWT) code.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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