Expose needs support for managing a service unit's open ports

Expose needs support for managing a service unit's open ports. This should implement the following scheme:

async methods

ServiceUnit.open_port(self, port, proto)
ServiceUnit.close_port(self, port, proto)
ServiceUnit.get_open_ports(self)
ServiceUnit.watch_ports(self)

with data stored in units/<service unit internal id>/ports, using the format
{ "open": [{ "port": PORT, "proto": PROTO}, ...}]}

Changes are immediate and use our standard retry approach to ensure appropriate merging.