Port life cycle with regards to locking

[reconbot]

I'm opening this issue to describe locking behavior and locking behavior with regards to the port lifecycle. I'd want to wait until #27 is closed before making a PR.

Ignoring how a port is opened or requested. A closed port should be able to be opened or reopened, and a port object that goes out of scope should be closed (if it's not already) to prevent a resource leak by the browser. A crashed browser will terminate effectively closing the serial port as well.

Open ports should (based upon the discussion in #35) be locked for exclusive use by the browser (Using OS primitives such as flock or no sharing mode in *nix and windows) and for exclusive use by a single port object in all browser contexts.

A closed port would not retain this exclusivity and would error upon reopening if another serial port object were to have opened the same port.

A later version of this spec might allow resource sharing with other processes outside the browser or other objects but that is out of scope of the initial spec.