Moved message verification into a separate thread pool.

api/net/sf/briar/api/protocol/VerificationExecutor.java

components/net/sf/briar/db/DatabaseExecutorImpl.java

components/net/sf/briar/protocol/ProtocolModule.java

...nents/net/sf/briar/protocol/VerificationExecutorImpl.java

.../sf/briar/transport/batch/BatchConnectionFactoryImpl.java

...net/sf/briar/transport/batch/IncomingBatchConnection.java

...t/sf/briar/transport/stream/IncomingStreamConnection.java

...t/sf/briar/transport/stream/OutgoingStreamConnection.java

...nents/net/sf/briar/transport/stream/StreamConnection.java

...f/briar/transport/stream/StreamConnectionFactoryImpl.java

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