Run the AS senders as background processes

[richvdh]

This should fix some "Starting db connection from sentinel context" warnings, and will mean we get metrics for these processes.

[richvdh]

Hopefully the test failures will get fixed by #4209 or #4213 or something

[codecov-io]

Codecov Report

Merging #4189 into develop will increase coverage by 6.89%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4189      +/-   ##
===========================================
+ Coverage    66.65%   73.55%   +6.89%     
===========================================
  Files          299      299              
  Lines        29798    29808      +10     
  Branches      4871     4872       +1     
===========================================
+ Hits         19863    21926    +2063     
+ Misses        8501     6455    -2046     
+ Partials      1434     1427       -7

Impacted Files	Coverage Δ
synapse/appservice/scheduler.py	86.71% <100%> (+30.36%)	⬆️
synapse/handlers/federation.py	61.72% <0%> (+0.1%)	⬆️
synapse/handlers/user_directory.py	71.03% <0%> (+0.3%)	⬆️
synapse/app/homeserver.py	57.28% <0%> (+0.33%)	⬆️
synapse/handlers/sync.py	92.05% <0%> (+0.44%)	⬆️
synapse/rest/client/v1/room.py	92.75% <0%> (+0.5%)	⬆️
synapse/rest/client/v1/presence.py	65.95% <0%> (+1.06%)	⬆️
synapse/push/httppusher.py	68.23% <0%> (+1.17%)	⬆️
synapse/handlers/directory.py	73.12% <0%> (+1.25%)	⬆️
synapse/handlers/profile.py	54.6% <0%> (+1.31%)	⬆️
... and 109 more

---

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[richvdh]

yay, the test failures went away.

[richvdh]

[I'm not entirely sure why codecov thinks I've changed 119 files]