Remove referral of "Gateway" in Apex Core and make the message bus mechanism pluggable

Currently we have referral to the "Gateway" in Apex Core code, especially in the attribute specs in Context.java.

This "Gateway" is actually functioning as a message bus that delivers data like STRAM events, tuple recording updates, stats updates, AutoMetrics data, and is not required for running an Apex application.

We should make this feature pluggable remove all references to anything specific to the "Gateway".