The method used by Commons IO to get the available disk space fails on
Android devices that lack a df binary - use the Android API instead.

Fixes issue #3612607.

To fix issue #3611966, KeyManagerImpl's handling of TransportAddedEvent
was made asynchronous. This made it possible for a thread to call
KeyManager.endpointAdded() before the KeyManager had asynchronously
handled the TransportAddedEvent from a previous call to
DatabaseComponent.addTransport().

The invitation and private messaging UIs are currently broken. Some key
rotation bugs were fixed; others may have been created (unit tests
needed). An encoding for private keys was added. Pseudonyms were moved
out of the messaging package and ratings were moved in.

The Tor plugin tests are consistently failing with the latest versions
of Silvertunnel and Spongy Castle; we may need to ship native Tor
binaries for Windows/Mac/Linux and use Orbot on Android.