Make client measure latency to the coordinator

[cmnord]

Addresses https://github.com/vuvuzela/vuvuzela/issues/43. Depends on https://github.com/vuvuzela/internal/pull/1.

Client sends ping messages regularly and updates its value for latency every time it receives a pong.

Test has the client ping every 10ms, then check that latency is 0 after 10ms and >0 after ~15ms.