Improve instance processing

[r4victor]

Fixes #1249
Fixes #1241

This PR makes several improvements to process_instances background tasks:

• Makes instance processing concurrent; releases the locks as soon as the instances are processes – should make instance processing more responsive.
• Fixes terminate_idle_instances taking global lock on instances that might cause runs stuck in terminating ([Bug]: The server stopped responding to certain requests after starting to terminate an instance #1241)
• Merges terminate_idle_instances background task into process_instances to remove any lock contention and make processing times more predictable.