StreamTransportPlugin into a superclass.

readerFinished() callback method to FilePlugin.

and close() to throw IOExceptions as well as DatabaseExceptions.

Added TestDatabaseModule so tests can specify their own DB
configuration. The modules are currently too tightly coupled - see
whether any dependencies can be removed.

The goal of the refactoring was to clean up the dependencies of
IncomingBatchConnection and OutgoingBatchConnection.

Also moved IO out of the lock in receiveOffer().

The return value indicates whether any batch IDs were written.
AckWriter.finish() and Database.removeBatchesToAck() are only called
if at least one batch ID was written.

encapsulation.

This should allow callers to calculate maximum packet sizes without
knowing the details of the transport protocol.

This allows a connection to know whether to try writing another batch
immediately, or to wait for an event from the DB.

Private messages have no group.

optional fields.

The initiator flag in the transport protocol makes this unnecessary by
linking the two sides of a stream-mode connection, making it
impossible for an attacker to replay the responder's side of a
different connection.

The flag is used to distinguish between the initiator and responder
directions of a stream-mode connection, allowing them to use the same
connection number without risking IV reuse.

The flag is also raised for batch-mode connections, which only have
one direction.