Extracting common headers from zmq_daq_core.h

A number of structures have common portions, extract the common header fields.

Fixed a spurious copy/paste error in comm_impl.cc.

Moved the zmq_recv from using a max number of dcus to using a max transmission size.

git-svn-id: https://redoubt.ligo-wa.caltech.edu/svn/advLigoRTS/trunk@4653 6dcd42c9-f523-4c6d-aada-af552506706e