feature request: websocket api including client and server

[liudongmiao]

There are similar issues for client: #1548
And there is a minimal websocket server in master branch since commit e831308, including 11+ commits after.

For websocket, we should think about how libevent would offer websocket.

• generate sec-websocket-key? requiers base64, used by client to generate, and server to validate.
• generate sec-websocket-accept? requires sha1, used by client to validate, and server to generate.
• handle websocket handshake? leave the sec-websocket-key / sec-websocket-accept for user code, or offer them in libevent?
  • IMO, libevent shouldn't offer base64 / sha1, user choose their own from mbedtls or openssl should be better.
• parse websocket frame? use bufferevent_filter_new with BEV_NEED_MORE is easy.
  • support mask? and custom fin, rsv, op, ...?
• build / make websocket frame?
  • support mask? and custom fin, rsv, op, ...?
• should 64-bits payload be supported?
  • if it's less than 2**16, libevent / user code can buffer such a small frame.
  • otherwise, libevent / user code should remember the mask key in the frame lifecycle.
• handle ping automatically?
• support custom extension?

IMO, if libevent choose to offer websocket, both client and server should be offered.

And when extra components were added in libevent, we should consider the quality. I never forget the issue in stackoverflow:

the extra components such as the http and dns servers suffered from bad implementation quality and resultant security issues

The comments were issued on 2012, and updated on 2017, and I don't agree it after a simple review for the http component I'm using.
However, we should consider for the extra component in libevent.