Optimize Redis-based heartbeat implementation

[zcahana]

The heartbeat operation is expected to be the most frequent API operation invoked on the Registry.
Today it is implemented using 3 round trips to Redis - read, write, expire - and all with a Write-lock acquired (see #288).

This can already be reduced to 2 operations: read, write+expire as a MULTI-EXEC operation.
Looking forward, we also need to look into using server side scripting (Lua) to implement this as 1 roundtrip.