[Gateway] set a default of 5m to `recover_after_time` when any to the `expected*Nodes` is set

The `recovery_after_time` tells the gateway to wait before starting recovery from disk. The goal here is to allow for more nodes to join the cluster and thus not start potentially unneeded replications. The `expectedNodes` setting (and friends) tells the gateway when it can start recovering even if the `recover_after_time` has not yet elapsed. However, `expectedNodes` is useless if one doesn't set `recovery_after_time`. This commit changes that by setting a sensible default of 5m for `recover_after_time` *if* a `expectedNodes` setting is present.

Closes #6742