rpc_tester: Collect latencies histogram
rpc_tester: Collect latencies histogram

Copied from the io-tester code. Each sending of an ECHO message notes
the time it took to send it and get back the result, then accumulates
it into a boost accumulator. In the end mean, some pXs and maximum
latencies are emitted as a part of YAML output.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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