Make SendZeroCopy independent of libzmq version

SendZeroCopy can be now places into the general zmq sources since the detection
of libzmq version is happening at compile time.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
Add SendZeroCopy tests and benchmarks

Added a test for SendZeroCopy and also a few benchmarks to compare it with the
regular Send function. The code of those benchmarks is probably not optimal yet,
there is some code that was simply copy-pasted.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>