Reimplement state synchronization in backend for nestable schedules

[ethanjli]

This PR prepares the backend for implementation of the Event Synchronization application-level protocol layer by refactoring the backend's implementation of the State Synchronization protocol layer (but without changing the State Synchronization protocol specification), specifically:

• By replacing the protocols.application.states.Synchronizer class with a more general protocols.application.states.SequentialSender class with more general functionality and without timing-related functionality, and a protocols.application.states.TimedSequentialSender class which wraps around SequentialSender with a constant send interval. Compare to #398, which performed the same refactor but did not provide a TimedSequentialSender class because the firmware only needed one root SequentialSender and one timer, for sending states to the Python backend; while the Python backend requires three SequentialSenders and three timers, for sending states to the firmware, the frontend, and the filesystem - so encapsulating timing-related functionality is useful in the Python backend.
• Otherwise, the concepts and schedule configurations for the State Synchronization implementation are the same as in #398.

I have fully removed the unit tests which were written for states.Synchronizer. We should hold off on writing unit tests for protocols.application.states, as the interfaces of the classes may undergo further changes for implementation of the Event Synchronization protocol layer.

[ethanjli]

@rohanpurohit For this PR, let's merge after you test this to look for any regressions - the backend (both ventserver.simulator and ventserver.application) should still behave the same as before this PR.

For records-keeping:

1. This project is licensed under Apache License v2.0 for any software, and Solderpad Hardware License v2.1 for any hardware - do you agree that your contributions to this project will be under these licenses, too? Yes
2. Were any of these contributions also part of work you did for an employer or a client? No
3. Does this work include, or is it based on, any third-party work which you did not create? No