Review WebSocket flushers

[sbordet]

Jetty version(s)
12.1.x

Enhancement Description
After #13034, review WebSocket flushers, in particular:

• Use Deflater.deflate(ByteBuffer) instead of deflate(byte[]), and honor the use of direct ByteBuffers also for deflate.
• Make sure that fields are reset and inflater/deflaters are re-pooled in case of exceptions.