Implements flush_async() and flush_finish() in EvdThrottledOutputStream

The asynchronous flush operation here does not actually do anything. It just
chains up with base stream's flush async.

The only reason to add this is to be able to clear the pending flag of the
stream, since the GOutputStream's default async_flush() does not do it.
Frees EvdThrottledOutputStream' throttle object GList with g_list_free_full()

instead of walking through the elements.
Makes EvdBufferedOutputStream to always call flush_finish() on base stream
Fixes variable name in assertion introduced by mistake
Adds WebSocket protocol implementation following RFC-6455

This new set of functions will deprecate evd-websocket-common.* and remove
the per-version implementations (00 and 08).

The implementation still lacks some details and checks, but is a fully working
start point.
Updates EvdWebsocketServer to use new protocol implementation
Removes WebSocket implementations for versions 00 and 08, and common code

The new WebSocket implementation is in evd-websocket-protocol.*
Changes to TRUE default 'standalone' property of EvdWebsocketServer
Removes unnecessary code block in request_handler() of EvdWebsocketServer
Considers several tokens in 'Connection' header of WebSocket handshake request

Firefox sends 'Connection: Upgrade, keep-alive' together.
Adds annotations to evd_websocket_protocol_bind()
Considers 'Connection' header to include several tokens in EvdHttpConnection
Adds new method get_transport() to EvdPeer
Adds new class EvdWebsocketClient (still WIP)

This is the high-level, class for creating client WebSocket connections.

A functional test is included. It is also WIP.
Completes implementation of peer validation in EvdWebsocketClient

The corresponding unit test is pending.
Removes unused methods dispose() and finalize() of EvdWebsocketClient
Autostarts TLS if WebSocket url scheme is 'wss' in EvdWebsocketClient
Closes the peer of a connection being removed from EvdWebsocketClient
Removes a status reason variable which is not used in EvdWebsocketClient
Adds set/get_tls_autostart() and set/get_tls_certificate() to EvdConnectionPool

Internal implementation of TLS upgrade is not ready yet.