Unregister sessions from server when their heartbeats time out.
Better error when releasing an unbuilt git sha
Make subscriptions handles reactively stop.

Although `handle.ready()` became false when the subscription was stopped, it didn't do so reactively, 
which led to some difficult to understand issues.
Added test case for reactive handle argument changing.
Explicitly test for sub arguments changing before ready.
Ensure the sub reactively goes unready when removed.

Subscriptions are removed from 3 points - handle.stop, nosub message, and _unsubscribeAll. We need to handle all three cases similarly.
Removed redundant checks for Deps' existence.