See issue #1608.

This is an old issue with Windows 7. The effect is that we see a latency
ramp on the first 500 messages.

* The ramp is unaffected by message size.
* Sleeping up to 100msec between sends has no effect except to switch
    off ZeroMQ batching so making the ramp more visible.
* After 500 messages, latency falls back down to ~10-40 usec.
* Over inproc:// the ramp happens when we use the signaler class.
* Client-server over inproc:// does not show the ramp.
* Client-server over tcp:// shows a similar ramp.

We know that the signaller is using TCP on Windows. We can 'prime' the
connection by doing 500 dummy sends. This potentially causes new sockets
to be delayed on creation, which is not a good solution.

Note that the signaller sends zero-byte messages. This may also be
confusing TCP.

Solution: flood the receive buffer when creating a new FD pair; send a
1M buffer and discard it.

Fixes #1608

This causes assertion failures after network reconnects.

Solution: allow EINVAL as a possible condition after read/write.

Fixes #829
Fixes #1399

Patch provided by Michele Dionisio @mdionisio, thanks :)

Cmake winci fixes

problem: random byte generation on windows got stuck in an infinite loop
solution: the failure test is incorrect. Change it

problem: not all configurations were built (and some were duplicated) in
Windows CI
solution: add all correct combinations to appveyor.yml

problem: cmake added a prefix of lib to libzmq, resulting in
liblibzmq.so
solution: set an empty prefix

CTest: add all sources in tests folder to CTest

TweetNaCL: add windows randombytes implementation

CMake: use libsodium + add Windows CI

problem: zmq_pollfd is not needed anymore when zmq_poller in place.

problem: ZMQ_POLLER doesn't support polling on out

enable static libraries on mingw

honor buffer reference count in zmq::shared_message_memory_allocator:…

problem: polling on multiple sockets is complicated

Fixed for issue  #1315

fix a typo in ypipe.hpp

Fixes issue #1556

Issue 1588 : Rollback in case send fails for multipart msg

create signaler::recv_failable()