New classification design.

Some proposals for a new classification design:

• replace the list+set by a multi-index
• replace the required-xxx by a more direct add-client-classes.
• add this new add-client-classes to host reservations as an alias of the existing client-classes (same entry with the same behavior for all objects which add a class to the query)
• complete the list of class evaluation points:
  • new points after the deferred unpack, pkt*_receive hook, etc
  • make clear in the doc that which a classification point is for:
    • dependency on a packet procession phase (e.g. KNOWN/UNKNOWN)
    • usage for the next packet processing step (e.g. subnet selection, pool guard, output option)
  • add an enum (vs a few flags) for the point where a class must be evaluated
  • add a meta-data with the value of its enum and make it visible to users
• same rules on dependency (use of member in expression):
  • no forward reference (the user class in a member clause must be already defined)
  • get the last classification point
  • perhaps a new built-in class for instance for the pkt*_receive hook
• document the way to switch from expired-* to this new stuff (but do not develop a tool to translate configurations)
• (next steps?) new uses of classes (e.g. lifetime), new expressions (e.g. in the response vs the query): in almost all cases this means new classification points