The default read strategy for HTTP/1 connections is now adaptive. It
increases or decreases the size of the read buffer depending on the
number of bytes that are received in a `read` call. If a transport
continuously fills the read buffer, it will continue to grow (up to the
`max_buf_size`), allowing for reading faster. If the transport
consistently only fills a portion of the read buffer, it will be shrunk.
This doesn't provide much benefit to small requests/responses, but
benchmarks show it to be a noticeable improvement to throughput when
streaming larger bodies.
Closes#1708
fd25129dc0