Added Consumer support to Writer, to avoid redundant copying.

api/net/sf/briar/api/serial/Consumer.java

api/net/sf/briar/api/serial/Writer.java

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

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

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

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

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

components/net/sf/briar/serial/ReaderImpl.java

components/net/sf/briar/serial/WriterImpl.java

components/net/sf/briar/transport/MacConsumer.java

test/net/sf/briar/FileReadWriteTest.java

test/net/sf/briar/serial/ReaderImplTest.java