rpc: do not destroy stream connection without stopping in case of negotiation failure
rpc: do not destroy stream connection without stopping in case of negotiation failure

A stream connection may fail in make_stream_sink before sink is
created. In this case a user has no way to wait for the connection to
stop, so it should be done internally, before returning.

Fixes #1137

scylladb/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar