Merge branch '167-namespaced-settings' into 'master'

ENH: Replaces transport config with namespaced settings

This MR contains the following:

* Adds a new guice module: SettingsManager, that abstracts the settings interface from the underlying database
* Uses SettingsManager to replace the db.getConfig/mergeConfig calls in the Settings activity
* Replaces the getSettings and mergeSettings calls in the database modules to include a namespace parameter. Updates the corresponding queries in the Jdbc module
* Removes the TransportConfig module, as it is not used anymore
* Updates the DuplexPlugin interface, to reflect this change in schemas and calls
* Updates the calls in the corresponding plugins, for both briar-android and briar-desktop
* Updates the hardcoded plugins in the tests to reflect these changes and updates the unit tests.

See merge request !65