Follow-up on #3019: limits are incompatbile with retry-on-startup

The limits library needs the lease manager initialized in dhcpX_srv_configured in order to check for JSON support and to do some recounting. When retry-on-startup is configured for the lease database, and a retry is triggered, the lease manager is not yet available, so an exception is thrown and the reconfiguration aborts, instead of actually retrying.

We should make limits compatible with retry-on-startup. Through some lazy recounting mechanism. @razvan's idea was a callback in LeaseMgrFactory that gets called on instantiation.