Accept connections asynchronously.

api/net/sf/briar/api/transport/ConnectionRecogniser.java

components/net/sf/briar/plugins/PluginManagerImpl.java

...ents/net/sf/briar/transport/ConnectionDispatcherImpl.java

...ents/net/sf/briar/transport/ConnectionRecogniserImpl.java

test/net/sf/briar/ProtocolIntegrationTest.java

test/net/sf/briar/db/H2DatabaseTest.java

test/net/sf/briar/plugins/PluginManagerImplTest.java

.../net/sf/briar/transport/ConnectionRecogniserImplTest.java

test/net/sf/briar/transport/ConnectionWriterTest.java

...f/briar/transport/batch/BatchConnectionReadWriteTest.java