storage_proxy: pass pending_endpoints by reference instead of by value
storage_proxy: pass pending_endpoints by reference instead of by value

This makes lifetime of dead_endpoints object more clear and move() also has its price.

Message-Id: <773c066d9ecbd291fd8529d03d07150126c1b083@scylladb.com>

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