[feature request] Increase the limit on pending client circuits with the number of onion services

I suggest to add a new torrc config variable:

MaxClientCircuitsPendingPerHS hsCircuitsPending

This will be in addition to the existing

MaxClientCircuitsPending staticCircuitsPending

The actual number of pending circuits then will be determined by the formula:

totalCircuitsPending = staticCircuitsPending + hsCircuitsPending*numHS

where numHS is the current number of hidden services.

This way, when the apps would open many hidden services, this will also increase the limit on pending circuits, and the possibility of running out of them will decrease.