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

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.

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().

encapsulation.

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

LockFairnessTest passes on Windows, Mac and Linux, so it's safe to use
ReadWriteLockDatabaseComponent on all those platforms. We can revisit
this issue for Android if necessary, but for now it's a waste of
effort to have two parallel implementations.