r11819: simplified the async rpc bind code a little.

r11819: simplified the async rpc bind code a little.

 - removed the struct dcerpc_request_state as all the state
   information is already available on the dcerpc_pipe structure, so just
   use that

 - added a single dcerpc_recv_data() handler for receiving packets
   from the transport layer. This then does the initial decoding of
   the dcerpc packet, and then looks at the packet type in order to
   work out who to dispatch it to. This should allow in-flight async
   rpc requests to still work while a new bind or alter context is
   happening

 - ensure that if the transport indicates the connection is dead that
   any in-flight bind or alter context requests are given an error

 - removed full_request_private and instead use separate bind_private
   and alter_private pointers

 - added a few comments for some bits I found hard to understand
(This used to be commit 938e372bffbee1982ad8268b83c43514fd2e5633)