cache_hitrate_calculator: do not ignore a future returned from gossiper::add_local_application_state
cache_hitrate_calculator: do not ignore a future returned from gossiper::add_local_application_state

We should wait for a future returned from add_local_application_state() to
resolve before issuing new calculation, otherwise two
add_local_application_state() may run simultaneously for the same state.

Fixes #4838.

Message-Id: <8d43f324cc25ff7306b9692d134854441a7602ec@scylladb.com>

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