Replay-detection window for HS INTRODUCE2 cells causes HS reachability failures

Currently, hidden services only accept a v3 INTRODUCE2 cell from a client if the timestamp it contains is within 30 minutes of the service's current time, so that the service doesn't need to keep entries in its replay-detection cache for very long. We should expand that window.

But in order to figure out how large the window should be, we need some statistics for how many entries are stored in a popular hidden service's replay cache. We should also investigate the typical lifetimes of HS descriptors and service-side introduction-point circuits, to find out whether we can remove the timestamp check entirely.