Implement itp dependent on number of tests by user

[vondele]

I'm considering to implement a change to the itp calculation (see also https://github.com/official-stockfish/Stockfish/issues/3234#issuecomment-735132503):

https://github.com/glinscott/fishtest/blob/52da7b7f221e8c78089a79f02d43b4cb67528bd4/server/fishtest/rundb.py#L504-L519

basically adjusting the itp by the number of active tests by the same user. If we call that number Nactive something like

if Nactive <= 2:
  itp *= 2

or

  itp /= max(1, math.pow(Nactive,1./3.))

However, I don't know how I can compute Nactive. @tomtor can you see how to do that easily?

[tomtor]

@vondele

https://github.com/glinscott/fishtest/blob/52da7b7f221e8c78089a79f02d43b4cb67528bd4/server/fishtest/rundb.py#L553

Scans all runs every minute. We could calculate nactive for each user there.