Rework isccc_ccmsg to support multiple messages per tcp read

Rework isccc_ccmsg to support multiple messages per tcp read

Previously, only a single controlconf message would be processed from a
single TCP read even if the TCP read buffer contained multiple messages.
Refactor the isccc_ccmsg unit to store the extra buffer in the internal
buffer and use the already read data first before reading from the
network again.

Co-authored-by: Ondřej Surý <ondrej@isc.org>
Co-authored-by: Dominik Thalhammer <dominik@thalhammer.it>