Make quota configurable for DoH

Currently, DoH shares the quota with TCP, which makes little sense anyway (see tcp-clients option), because of the nature of interaction of DoH clients: they tend to keep idle opened connections for longer periods of time, preventing the TCP and TLS client from being served.

Because of these differences, it makes sense for DoH to have a separate quota facility. Also, it makes sense to make the number of streams per connection configurable as well, as these are treated as virtual connections by the code.

See !5036 (merged) for additional details.