Prepared message cache for compression, websocket upgrade performance improvements

[FZambia]

Add EXPERIMENTAL prepared message cache for compression scenario:

// PreparedMessageCacheMaxSize when greater than zero enables caching of WebSocket prepared
// messages and sets the maximum size of that cache in bytes. When cache is on – Centrifuge
// uses prepared WebSocket messages for connections with compression. This generally
// introduces overhead but at the same time may drastically reduce compression memory
// and CPU spikes during broadcasts. See also BenchmarkWsBroadcastCompressionCache.
// This option is EXPERIMENTAL, do not use in production. Contact maintainers if it
// works well for your use case, and you want to enable it in production.
PreparedMessageCacheMaxSize int64

Also, this PR has several tweaks in websocket package to reduce number of allocs during Upgrade: from 9 to 3.