implement transport limit for public visors

[0pcom]

There is no apparent limit on the number of transports that a public visor will accept.

However, there is a definite limit of the number of transports a visor can have. (approx. 250 from my tests)

There should be a limit on the number of transports that a public visor will accept specified in the visor config, with a sensible default value which is lower than the maximum number.

When this limit is reached, the public visor should de-register it's entry from the public visors service discovery so that no additional transports will be created to that public visor.

...

It would make sense to limit the number of connections to any given proxy or VPN server via similar mechanism ; however the issue for public visors is more urgent.