Customizable learn-address interval

[adamobringer]

It would be uber-useful to us if we could specify a customized learn-address interval, regardless of resource penalty via a variable such as 'learn-address-interval n'. Applicable use would be in a situation where we're establishing DNS records on connection via IXFR, and need to ensure that on disconnect, the old record(s) are completely nuked. With current behavior, this cleanup executes on it's own timer, and within that time frame, something else may connect and obtain the IP address that the previous device has. Cleanup then kills both records. CCD is not an option for us, since our OVPN solution is load balanced in the cloud (session server / server clustering would be incredible) and relying solely on 5-tuple load balancing logic to persist sessions.