Use get-ports for local server ports

[gmaclennan]

Describe the idea
Check for port availability before listening on local servers

Additional context
In mapfilter-desktop we use get-ports to find an available local port, this avoids potential conflict with existing in-use ports.

[okdistribute]

This should only be used for mapeo-server, not osm-p2p-server because custom imagery in iD editor depends upon the port being static (persistent) between app restarts.

[gmaclennan]

Good point, we should change the way custom imagery is handled at some point, but in the meantime this is good to bear in mind.