Make worker asynchronous and stoppable

[callumforrester]

Changes:

• Change worker API from run_forever to stop and start
• Make start run worker in its own thread

[codecov]

Codecov Report

Merging #145 (0c22c87) into main (28042db) will increase coverage by 8.33%.
The diff coverage is 94.82%.

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   74.62%   82.95%   +8.33%     
==========================================
  Files          38       39       +1     
  Lines        1056     1103      +47     
==========================================
+ Hits          788      915     +127     
+ Misses        268      188      -80     

Impacted Files	Coverage Δ
src/blueapi/service/app.py	40.35% <0.00%> (ø)
src/blueapi/worker/reworker.py	79.75% <95.23%> (+46.42%)	⬆️
src/blueapi/core/__init__.py	100.00% <100.00%> (ø)
src/blueapi/core/bluesky_event_loop.py	100.00% <100.00%> (ø)
src/blueapi/worker/multithread.py	100.00% <100.00%> (+33.33%)	⬆️
src/blueapi/worker/worker.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[callumforrester]

@RAYemelyanova I think this this is ready now

[rosesyrett]

looks good, I've got a couple of comments about small things.