Major rework:
- Change priority in `DeviceProxy` and `AttributeProxy` constructors.
  If TestContext inactive, use TRL (Tango Resource Locator) as provided by caller.
  If TestContext active and short TRL, replace with fully-qualified TRL, using
  TestContext host and port.  If replaced TRL fails, try short TRL.
- Improve error reporting if these constructors fail, even after retying.
- With the `Group` object, we still don't do the retry, as there
  isn't a convenient place to put that code in PyTango.
- When TestContext is synchronising the port between (possible) subprocess and parent, no need for background thread.
- TestContext doesn't need to determine the host again in `pre_init` - it is already known
  before we start the device server.
- Add a friendly name to the TestContext DS launcher process/thread to make
  debugging easier.
- Startup state of device server can be represented with just Exception object.
  In Python 3 this includes the traceback info, so no need to handle it separately.
- No need to mess with environment variables, we can just use a global variable.
- Use leading underscores for utility functions.  Even though they are used in other modules, so should be public, we don't want PyTango users to use them.
- Change URI abbreviation to TRL.
- Add some tests for nested `DeviceTestContext` - this is not recommended usage,
  but we test it since it used to be possible.  Update documentation as well.
- Minor changes to multidevicetestcontext example - Python types preferred.