Fix HTTPBodySequence for large requests (adds AsyncBufferedPrefixSequence)

[swhitty]

fixes #139

HTTPRequest.bodySequence is aways backed by one of the the following concrete types:

• AsyncSharedReplaySequence: Sequence that can be safely iterated multiple times by one or more consumers concurrently. It is buffered in memory and is used for all requests with body less than 2_097_152 bytes

  FlyingFox/FlyingFox/Sources/HTTPServer+Configuration.swift

  Line 46 in fe9437d

  sharedRequestReplaySize: Int = 2_097_152,

• AsyncSocketReadSequence Sequence that is backed directly by the socket itself. Used for all requests with body greater than 2_097_152 bytes.

A bug exists when AsyncSocketReadSequence is used, because the sequence is only terminated when the underlying socket is closed, the sequence hangs when all bytes are read but the socket remains open.

This PR adds AsyncBufferedPrefixSequence which wraps the underlying AsyncSocketReadSequence terminating the sequence when all bytes have been read.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.83%. Comparing base (fe9437d) to head (4231165).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
- Coverage   94.85%   94.83%   -0.02%     
==========================================
  Files          60       61       +1     
  Lines        3943     3971      +28     
==========================================
+ Hits         3740     3766      +26     
- Misses        203      205       +2     

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