Classifier: Track address prefixes.

Classifier: Track address prefixes.

Add a prefix tree (trie) structure for tracking the used address
space, enabling skipping classifier tables containing longer masks
than necessary for an address field value in a packet header being
classified.  This enables less unwildcarding for datapath flows in
parts of the address space without host routes.

Trie lookup is interwoven to the staged lookup, so that a trie is
searched only when the configured trie field becomes relevant
for the lookup.  The trie lookup results are retained so that each
trie is checked at most once for each classifier lookup.

This implementation tracks the number of rules at each address prefix
for the whole classifier.  More aggressive table skipping would be
possible by maintaining lists of tables that have prefixes at the
lengths encountered on tree traversal, or by maintaining separate
tries for subsets of rules separated by metadata fields.

Prefix tracking is configured via OVSDB.  A new column "prefixes" is
added to the database table "Flow_Table".  "prefixes" is a set of
string values listing the field names for which prefix lookup should
be used.

As of now, the fields for which prefix lookup can be enabled are:
- tun_id, tun_src, tun_dst
- nw_src, nw_dst (or aliases ip_src and ip_dst)
- ipv6_src, ipv6_dst

There is a maximum number of fields that can be enabled for any one
flow table.  Currently this limit is 3.

Examples:

ovs-vsctl set Bridge br0 flow_tables:0=@N1 -- \
 --id=@N1 create Flow_Table name=table0
ovs-vsctl set Bridge br0 flow_tables:1=@N1 -- \
 --id=@N1 create Flow_Table name=table1

ovs-vsctl set Flow_Table table0 prefixes=ip_dst,ip_src
ovs-vsctl set Flow_Table table1 prefixes=[]

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>