Dynamically adjust max number of leases

This change dynamically adjusts the maximum number of leases based on
the size of the subnets associated with a network.  The upper bound is
limited by a configurable option to keep the max reasonable and prevent
denial of service.

Closes bug: 1225200

Change-Id: I75c3907bcf45cd991eadf5dd8c8ad7f1eaab3c85
Merge "Dynamically adjust max number of leases"