core code that helps with rate limiting

Part of #562 (closed).

• b2a960b4 add scoped_lock. Needed to lock multiple mutexes at once. thread
• 169df5a0 use SubnetID from subnet_id.h in Lease. Extraneous, but can we keep it?
• 03452520 enable range-based for loops on ClientClasses. The for loop decides if the const or non-const version is called based on the constness of the object that is being iterated.
• 7e47dc28 template adapters to existing qualified names. Hoping to kickstart the further use of templates for common code between v4 and v6 functionality.
• 9187be72 mention mark and argument in logger error
• da1cc865 add an ordered_unique index to ClientClassContainer
• 6c21b323 document rate limiting
• 2ee3eaea add ChangeLog entry