(sync) Reliable event subscriptions or hooks for documents

[Frando]

Users will want to reliably react to document updates - eg to run compute or populate secondary indexes. We provide a event subscription API; however if this subscription is set up over the RPC or IPC this takes a few ticks, and thus may miss doc updates especially when the node is started.

We should look into either adding a hook feature or allowing to setup a catch-all subscription that can be created directly with node startup.

#1495 improves subscriptions a bit but they are still an async call away, so not reliable to catch all events.