Add WebSocketConfig::read_buffer_size default 128 KiB

[alexheretic]

• Add WebSocketConfig::read_buffer_size default 128 KiB.
• Make WebSocketConfig non-exhaustive & add builder style construction fns.
• Remove deprecated WebSocketConfig::max_send_queue.

Note this also doubles the read buffer len in #465 from 64 -> 128k providing a boost in bench perf, since the bench has 100k messages ready to go.

The bench is a bit artificial to deduce the best default, but 128k is the current default for the write buffer so seems a good place to start.

read+unmask 100k small messages (server)
                        time:   [7.5623 ms 7.5700 ms 7.6009 ms]
                        change: [-3.5836% -3.0376% -2.4881%] (p = 0.07 > 0.05)

read 100k small messages (client)
                        time:   [7.1301 ms 7.1658 ms 7.1747 ms]
                        change: [-4.4713% -2.4551% -0.3652%] (p = 0.13 > 0.05)