Datastore aggregation query, composite filter and multiple database support

[owainhunt]

Adds support for aggregation queries (count, sum and average) against Datastores.

Also:

• Adds support for OR operator on composite datastore filters (previously only supported AND)
• Adds support for passing databaseId to datastore functions (previously only supported default database)
• Fixes LookupResponse model so missing entities can be correctly decoded (previously this would fail, as a missing entity does not have the same properties as one that exists)