TCPRewriter: add perform_mapping handler

This handler takes in a flow as a space-separated

	saddr sport daddr dport

and looks for a forward mapping for that flow.  If found, it returns the
rewritten flow in the same format.  This is useful when using a TCPRewriter
to divert traffic to a proxy in userland.  It allows the proxy to figure out
the original destination for the flow, similar to SO_ORIGINAL_DST for
Netfilter or SIOCGNATL for IP-Filter.