Use client helper in existing clients

Use the new ClientHelper to reduce boilerplate in existing clients. Add a BdfMessageValidator superclass for clients that format their messages as BDF lists and their metadata as BDF dictionaries (which all existing clients do).

See merge request !115

BDF improvements

* Change the semantics of the BdfList/Dictionary methods with default values so they return the default value if the value is missing, or throw an exception if it the value's present but has the wrong type - this is more useful for validators, which have to deal with optional values without accepting values of the wrong type
* Reuse BdfReader/Writer for MetadataParser/Encoder so we don't have two implementations of BDF


See merge request !117

Helper class to reduce client boilerplate

* Renamed BdfReader methods for consistency with BdfList/BdfDictionary
* Added readList() and readDictionary() methods to BdfReader
* Added ClientHelper to reduce boilerplate when converting messages and metadata to and from BDF
* Moved PrivateGroupFactory to the same package as ClientHelper


See merge request !114

Unit tests for ValidationManagerImpl

Unit test for the validation manager. I also changed the way the validation manager loads unvalidated messages - instead of using a single DB task to load all unvalidated messages, it loads a list of message IDs and then loads each message in a separate task. This prevents the DatabaseExecutor from being blocked by a long-running task if there are lots of messages to validate.

See merge request !113

Add contact and transport keys in the same transaction

This avoids a potential problem where the app crashes after adding the contact but before adding the transport keys, leaving the contact unusable.

See merge request !112

Convert BDF types in lists and dictionaries



See merge request !111

Each task is added to the back of the executor's queue, allowing tasks from other callers to be interleaved.