Add ZMQ_CONFLATE socket option

Minor doc & assert fixes

Optimized zmq::msg_t for constant messages

Attempt to fix disconnect not respecting linger

Port tests to windows and add to cmake build.

Looks like linger is honoured properly, but shutting down the session
causes the pipe termination to come from that side - because the local
pipe then shuts down right away it seems to trigger a terminated on the
other end instead of waiting. This way we trigger the termination from
the local end and then terminate the session.

Add zmq_send_const for constant data

ZMQ_CONFLATE option is passed to pipepair() which creates a usual
ypipe_t or ypipe_conflate_t and plugs it into pipe_t under a common
abstract base.

Add simple double buffer implementation tailored to handle msg_t,
i.e invoke msg_t::close instead of destructor and so on.
Seems to me mutex is good enough at this point.

Required for memmove(), memcpy()

Fix compile error on windows.

Rename ZMQ_REQ_SEND_RESETS -> ZMQ_REQ_STRICT.

It defaults to enabled. Switch it off to be able to send a new message
before the previous reply has been received.

Add ZMQ_REQ_SEND_RESETS option.

Added note to clarify policy for embedded apps

* Fixes a terminate() call on a dangling pointer in the SEND_RESETS
  case.
* Fixes recv_reply_pipe() never receiving a message once the pipe it is
  waiting on is terminated.

This allows making a new request on a REQ socket by sending a new
message. Without the option set, calling send() after the first message
is done will continue to return an EFSM error.

It's useful for when a REQ is not getting a response. Previously that
meant creating a new socket or switching to DEALER.

Add ZMQ_REQ_REQUEST_IDS option.

* Documentation:
The default behavior of REQ sockets is to rely on the ordering of messages
to match requests and responses and that is usually sufficient. When this option
is set to 1, the REQ socket will prefix outgoing messages with an extra frame
containing a request id. That means the full message is (request id, 0,
user frames...). The REQ socket will discard all incoming messages that don't
begin with these two frames.

* Behavior change: When a REQ socket gets an invalid reply, it used to
  discard the message and return EAGAIN. REQ sockets still discard
  invalid messages, but keep looking at the next one automatically
  until a good one is found or there are no more messages.
* Add test_req_request_ids.

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