flow, tcp: Generalise TCP hash table to general flow hash table

Move the data structures and helper functions for the TCP hash table to
flow.c, making it a general hash table indexing sides of flows.  This is
largely code motion and straightforward renames.  There are two semantic
changes:

 * flow_lookup_af() now needs to verify that the entry has a matching
   protocol and interface as well as matching addresses and ports.

 * We double the size of the hash table, because it's now at least
   theoretically possible for both sides of each flow to be hashed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

flow_table.h

@@ -146,7 +146,4 @@ void flow_activate(struct flow_common *f);
 #define FLOW_ACTIVATE(flow_)			\
 	(flow_activate(&(flow_)->f))
 
-uint64_t flow_hash(const struct ctx *c, uint8_t proto, uint8_t pif,
-		   const struct flowside *side);
-
 #endif /* FLOW_TABLE_H */