Make the 2-second timeout parameter configurable

[Baron Schwartz]

Currently there is a hard-coded 2-second timeout that discards a session if activity is not seen for 2 seconds. This makes it impossible to measure queries longer than 2 seconds. Convert this into a command-line option and make the default 3600 seconds instead.

The worst-case wrong behavior is that an inbound packet is seen, the outbound response is not seen, another inbound packet appears but is not seen, and then we see the outbound response and associate it with the first inbound packet. This is unlikely, relative to the number of cases where at least one of the intermediate packets is seen.

The risk is that we accumulate stale sessions for up to an hour. However, these only use a small amount of memory, and should not be a problem. By making this parameter configurable, we can experiment with that.