Some tweaks for the device list updater

[S7evinK]

This makes the following changes:

• Adds two new metrics observing the usage of the DeviceListUpdater workers
• Makes the number of workers configurable
• Adds a 30s timeout for DB requests when receiving a device list update over federation

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (32f7c4b) 65.52% compared to head (7d00753) 65.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3251      +/-   ##
==========================================
+ Coverage   65.52%   65.54%   +0.02%     
==========================================
  Files         507      507              
  Lines       57198    57209      +11     
==========================================
+ Hits        37481    37500      +19     
+ Misses      15877    15873       -4     
+ Partials     3840     3836       -4     

Flag	Coverage Δ
unittests	49.53% <47.05%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cmd/dendrite-demo-pinecone/monolith/monolith.go	77.55% <100.00%> (ø)
cmd/dendrite/main.go	62.23% <ø> (-0.27%)	⬇️
setup/config/config_userapi.go	100.00% <100.00%> (ø)
userapi/userapi.go	72.44% <100.00%> (ø)
userapi/consumers/devicelistupdate.go	44.18% <0.00%> (-5.82%)	⬇️
userapi/internal/device_list_update.go	74.68% <50.00%> (-0.77%)	⬇️

... and 10 files with indirect coverage changes

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