Problem: messaging is only available through the server

[yrashk]

It is currently impossible to do any messaging-related functionality via pumpkindb_engine because SUBSCRIBE and UNSUBSCRIBE are essentially shims implemented by the server.

This severely limits embedding scenario as it would only have access to the post-execution stack but won't be able to send data during script execution (useful in cursor iteration, etc.)

Proposed solution: define a SubscriptionManager (or any better name) trait that would handle subscription mapping and delivery and pass it through to mod_msg (to be created).

It would also be a good time to define better and wider semantics for messaging. Right now SEND is fully sync (to make the current subscription scheme work).