gstreamer/gst/tcp

This part of the documentation is for the new tcp elements:
- tcpclientsrc
- tcpclientsink
- tcpserversrc
- tcpserversink
                                                                                
TESTS
-----
Use these tests to test functionality of the various tcp plugins

* server: nc -l -p 3000
  client: nc localhost 3000
  everything you type in the server is shown on the client
  everything you type in the client is shown on the server

* server: nc -l -p 3000
  client: gst-launch tcpclientsrc protocol=none port=3000 ! fdsink fd=2
  everything you type in the server is shown on the client

* server: nc -l -p 3000
  client: gst-launch fdsrc fd=1 ! tcpclientsink protocol=none port=3000
  everything you type in the client is shown on the server

* server: gst-launch tcpserversrc protocol=none port=3000 ! fdsink fd=2
  client: gst-launch fdsrc fd=1 ! tcpclientsink protocol=none port=3000

* server: gst-launch fdsrc fd=1 ! tcpserversink protocol=none port=3000
  client: gst-launch tcpclientsrc protocol=none port=3000 ! fdsink fd=2
  -> Received first buffer without caps set

TODO
----
- implement DNS resolution

multifdsink
-----------
- operation:
  - client fd gets added when "add" signal gets emitted on multifdsink
  - signal handler creates a GstTCPClient structure, adds it to ->clients,
    and adds the fd to ->fd_hash, then emits client-added
  - client 

  - when a buffer comes in:
    - the _render vmethod puts the buffer on the global queue
    - and increases bytes_to_serve
    - (currently it sets streamheaders, but since this is treated globally
       this is wrong - clients can be at different positions in the stream)

  - when a client issues a write (ie requests data):
    - when using GDP, if no caps sent yet, send caps first, then set caps_sent
    - if streamheader buffers, and we haven't sent yet to this client,
      send current streamheader buffers, then set streamheader_sent
    - send out buffers