Joshc slac/message handling

[joshc-slac]

Still building out bones for this thing. Goal of this PR is to allow for requests to be plumbed all the way to generated trees. This gets us all the way up to but not including tree generation.

Added test coverage for the Worker object as well as new PriorityQueue object.

Updated Sequencer main object to launch threads required for runtime specifically:

• message handler thread: parses requests into the tree structure that will actually be ticked
• sequence server thread: GRPC process that accepts reqests
• main thread: thing that will tick jobs

I added a PriorityQueue object to helpers as this is the standard way to ensure you sequencer chooses the correct task to begin execution on . In the future we will need to make sure that when it receives a higher priority task than the one currently executing it pauses current execution to field the more critical task. This is enabled in the current architecture.

[joshc-slac]

Add test coverage for SequenceServer and SequenceClient