Subnet Cache hashtable improvements

Subnet Cache hashtable improvements

 - inline & staticly allocated hash table
 - increased hashtable size (32bit: 1024, 64bit: 65536)
 - re-arrange subnet members
 - Add key type
 - reduce clearing of hash table
 - cleanup key pointer operations
 - removed unused hash_search_or_insert
 - add open addressing to hash table
 - type specific hash functions & hash seeding
 - no collisions for 32bit os
 - implement cache flush by SUBNET_MAC