Buffer HTTPRequest.body < 4096 bytes as Data

[swhitty]

HTTPRequest currently provides access to the body via HTTPBodySequence, this sequence is currently backed by 2 concrete sequences:

• 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.

This PR adds a 3rd type that can be used for small requests:

• Data / AsyncBufferedCollection is now used for small requests with a body less than 4_096 bytes.