Function poll

Polls the store for committed events after the factory's watermark. This should be the entry point to reactive service's event handlers.

Note: Polls are serialized in order to avoid competing consumers from processing the same events more than once, but consumers must be idempotent to accomodate potential failures and timeouts.

Using a basic leasing strategy to get this working (distributed lock persisted in the store as subscriptions)

TODO: improve with Redis or some other efficient mechanism