The Merger and executor APIs have a lot in common, but they behave
slightly differently. A merger needs to sometimes return results.
An executor needs to have separate queues for zones and be able to
pause or cancel jobs.
This refactors them both into a common class which can handle job
state changes (like pause/cancel) and return results if requested.
The MergerApi can subclass this fairly trivially.
The ExecutorApi adds an intermediate layer which uses a
DefaultKeyDict to maintain a distinct queue for every zone and then
transparently dispatches method calls to the queue object for
that zone.
The ZK paths for both are significantly altered in this change.
Change-Id: I3adedcc4ea293e43070ba6ef0fe29e7889a0b502
a729d6c6e8