atomic lease limits

Make the checking of lease limits atomic to the lease allocation process, and thus resulting in a hard limit cap, as outlined below:

• Add the limits to the lease candidate's user context under path ISC.limits in the leaseX_select callout.
• Add before-event triggers on the lease tables in MySQL and PostgreSQL that check the limits and prevent the subsequent INSERT or UPDATE statement if a limit is exceeded. If the INSERT or UPDATE is carried out, ISC.limits is removed from the user context.
• Signal the event of reaching a limit to the lease manager which logs its details.
• Make sure the event is properly handled as a frequent application logic event in the calling contexts (e.g. allocation engine, HA service, lease_cmds), as opposed to a technical failure which can disrupt the usual service or can be costly in terms of performance.