chore: improve pubsub

[glevco]

Motivation

This PR resolves #552. There are 3 goals:

1. Improve typing of code using Twisted reactors
2. Change tests to use a MemoryReactorClock instead of a Clock, so it's compatible with the reactor used in production (that is, it implements the IReactorCore, IReactorTime, and IReactorTCP Twisted interfaces. Clock only implements IReactorTime)
3. Remove PubSub support for reactors that don't implement IReactorCore

This was done by leveraging Python protocols to represent an intersection of Twisted interfaces, which allows us to better represent the reactor type. By doing this, we were able to change tests to use a reactor that is more similar to the one used in production, as previously we were using a Clock, that behaved differently than the production reactor during full node initialization. This also allows for simplifying PubSub, as it had a specifc implementation for dealing with Clock.

Acceptance Criteria

• Create new ReactorCoreProtocol, ReactorTimeProtocol, and ReactorTCPProtocol classes stubbing the respective Twisted interfaces, IReactorCore, IReactorTime, and IReactorTCP
• Create new ReactorProtocol that implements the 3 protocols above, effectively creating a type that represents an intersection of Twisted interfaces, and cast the Twisted reactor to that protocol (this resolves a XXX comment in hathor/util.py)
• Update PubSub to improve its code, also removing support for "non-running" reactors (Clock)
• Remove the ReactorThread enum as it's not used anymore
• Change the Clock to MemoryReactorClock in tests (this resolves a XXX comment)
• Change HeapClock to MemoryReactorHeapClock on simulator
• Improve verified_cast() utils function and move it to its own file

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Merging #694 (aa53857) into master (29d9ec3) will decrease coverage by 0.10%.
Report is 4 commits behind head on master.
The diff coverage is 95.72%.

❗ Current head aa53857 differs from pull request most recent head 3561317. Consider uploading reports for the commit 3561317 to get more accurate results

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
- Coverage   84.70%   84.60%   -0.10%     
==========================================
  Files         252      258       +6     
  Lines       21801    21783      -18     
  Branches     2954     2943      -11     
==========================================
- Hits        18467    18430      -37     
- Misses       2692     2715      +23     
+ Partials      642      638       -4     

Files Changed	Coverage Δ
hathor/debug_resources.py	58.41% <33.33%> (-0.35%)	⬇️
hathor/utils/zope.py	93.75% <93.75%> (ø)
hathor/builder/cli_builder.py	73.82% <100.00%> (-0.67%)	⬇️
hathor/p2p/sync_v1/agent.py	85.07% <100.00%> (+0.06%)	⬆️
hathor/p2p/sync_v2/streamers.py	88.48% <100.00%> (+0.07%)	⬆️
hathor/pubsub.py	96.00% <100.00%> (ø)
hathor/reactor/reactor.py	100.00% <100.00%> (ø)
hathor/reactor/reactor_core_protocol.py	100.00% <100.00%> (ø)
hathor/reactor/reactor_protocol.py	100.00% <100.00%> (ø)
hathor/reactor/reactor_tcp_protocol.py	100.00% <100.00%> (ø)
... and 5 more

... and 13 files with indirect coverage changes