zebra: fix zapi msg debugging dumps

zebra: fix zapi msg debugging dumps

When we switched to a pthread per client, we lost the ability to
correlate zapi message debugs with their handlers in zlog, because the
message was logged when it was read off the zapi socket and not right
before it was processed. Move the zapi msg hexdump to happen right
before we call the message handler.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>