use a file.

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.

If it's not possible to free up any additional space, exit rather than
hanging. In future we should warn the user before exiting.

The new method checks whether the parent is present in the database
and belongs to the same group, so separate methods for those checks
have been removed.

Otherwise an exception may be thrown if an offered message is no
longer in the DB.

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.

The locks were being held across all transactions anyway, so there was
no granularity advantage to using multiple transactions.