keep track of active users

[fisx]

Related: #915.

we want to have this:

type ActiveSessions = Set (ThentosSession, TimeStamp)
-- store ActiveSessions in AulaData

bump :: ThentosSession -> Action ()
gc :: Action ()
size :: Action Int

everything else happens under the hood in Persistent.Pure.

[fisx]

i forgot: there has to be a constant somewhere that determines how old is stale (in TimeSpan).

[fisx]

also, possibly even more relevant: we need maximum request time in the window of the last TimeSpan. does ekg help us with that?

[fisx]

definitely relevant: https://hackage.haskell.org/package/wai-middleware-metrics

[fisx]

#951 has the code to keep track of logged in users, but if nobody ever logs out, that does not say much about the number of active users. anyway, both figures are interesting. we'll close this for now.