used for main_dispatcher only in this patch.
Dispatcher is meant to be used for Main<->any low frequency messages.
It's interface is meant to include the red_dispatcher usage:
fixed size messages per message type
some messages require an ack
Some methods are added to be used by RedDispatcher later:
dispatcher_handle_read - to be called directly by RedDispatcher epoll
based loop
dispatcher_set_opaque - to be set from red_worker pthread
dispatcher_init - allow NULL core as used by red_worker
Read and Write behavior:
Sender: blocking write, blocking read for ack (if any).
Reader: poll for any data, if such then blocking read for a
message_type and following message. repeat until poll returns
with no pending data to read.
FDO Bugzilla: 42463
