rpc_test: fix cancel test to not depend on timing.
rpc_test: fix cancel test to not depend on timing.

sleep() may end before packet is even sent, so use future to signal that
rpc server handler is running.

Message-Id: <39f89f59518c316dd89b093ee50a10e27f6e09ca@scylladb.com>

dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,kjniemi/seastar,avikivity/seastar,kjniemi/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,kjniemi/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,scylladb/seastar