limits library extensions for lease limiting

Add lease limiting capabilities to the limits hook library as outlined below:

• Add the ability to configure lease limits.
• Add the packet's client classes to the lease candidate's user context under path ISC.client-classes in the leaseX_select callout.
• Use the LeaseMgr::checkLimitsX() functions in the leaseX_select callout. Set NEXT_STEP_DROP if the limit is exceeded.