Set a sensible 3PID lookups validity period

[maxidorius]

When computing the result of a 3PID lookup, we set a validity end time far in the future for the results. This is a problem for clients/servers that may implement actual caching, or for ourselves if we want to adding caching later for federated results.

We need to make this configurable with a sane default value like 1 day.

[maxidorius]

After evaluation, we do not have a good, central place in code where this could be set/enforced, we'll therefore only set a sensible validity period of 1 min before and 5 min after the lookup.