Sequencer app refactor

[ethanoroshiba]

After offline discussion, I have put together a list of issues which will hopefully improve our code quality along with outside readability in the sequencer app. The list is in order of which things should be accomplished first:

Tasks

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1. sequencer: move all ActionHandler impls to a central location #1657
   code-quality sequencer +2
   
2. sequencer: refactor app #1785
   refactor sequencer +2
   
3. rename and refactor begin_block and end_block #1809
   code-quality sequencer +2
   
4. Non-specific cleanup of app after all of this, likely consolidating many of the methods.

┆Issue Number: ENG-1007