CollectionSync: split transaction into multiple smaller transactions

The transaction actually starts a database transaction on Akonadi side and
it turns out that DB backends don't handle transactions with thousands of
INSERT statements very well (we noticed a big performance drop when executing
the queries).

This patch splits the one big transaction into smaller transactions of 100
operations (CollectionCreateJob). As result, creating 100,000 collections
takes much less time.

REVIEW: 119491