spawning too many Networks makes Network::new() fail

[japaric]

this is because the available IP address space gets exhausted. we should

• reduce the address space of each Network
• add a mechanism to make Network::new wait if there are too many other live Networks (e.g. a semaphore) or there's no address space available (e.g. a retry mechanism with exponential back-off)

[japaric]

The containers and the networks are not cleaned up when the tests are run via cargo watch because changes in files make cargo watch kill the last cargo test process. when the cargo test process and its children, which includes the test runner process, are killed the Container and Network destructors are not run (the destructors are in charge of removing the containers and networks).

This should be documented in the README and we should advise people to not run tests through cargo-watch