If no messages are added to a batch, don't call BatchWriter.finish() -

this allows the caller to avoid creating an empty packet by delaying
creation of the packet's header and trailer until something's written
to the packet's body. Changed the return semantics of
DatabaseComponent.generateBatch(ContactId, BatchWriter,
Collection<MessageId>) so that the IDs of messages considered for
inclusion in the batch but no longer sendable are also returned - this
allows the caller to remove them from the set of requested IDs.