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.

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.

checking whether the contact was still in the DB.