Propagate /v1/queue/empty to clients

[gferon]

This is a much smaller API breakage than I expected it to be and will be a massive quality of life improvements for clients of presage. It can also be used to properly wait for contacts sync.

Closes #94

[rubdos]

Android has refactored this API too, and there they return lists of Envelopes in batches. They also have a system in place where Envelopes can be acknowledged, instead of acking them in the stream itself.

That said, I think this design is perfectly fine.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (aa84243) 3.71% compared to head (400b043) 3.71%.

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #253      +/-   ##
========================================
- Coverage   3.71%   3.71%   -0.01%     
========================================
  Files         38      38              
  Lines       2824    2825       +1     
========================================
  Hits         105     105              
- Misses      2719    2720       +1     

Files	Coverage Δ
libsignal-service/src/websocket.rs	0.00% <0.00%> (ø)
libsignal-service/src/messagepipe.rs	0.00% <0.00%> (ø)

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