Support for same port on multiple interfaces

This commit improves upon the PortMapper and PortAllocator classes by changing
their internal data structures for port allocations to use a string rather than
a single integer. This string holds the network interface address as well as the
port number. This solves a previous problem where a port would be incorrectly
reported as being in use because it had been allocated for a different interface.

I've also added a basic test case for the PortMapper class, and extended the
existing test case for PortAllocator. In the case of PortMapper, this is done
by handing it a stub function for creating proxies rather than an actual
implementation.
Merge pull request #2796 from EmilHernvall/master

Support for same port on multiple interfaces