improve couchdb change sync behaviour based on a configured sync strategy

[tomwwinter]

Current implementation:

• loads all _couch changes and emit DocumentChangeEvents

Improvement:

• configure behaviour based on needs

possible sync options:

• use latest available revs (first time start)
• set a timestamp or a existing rev as starting point in configuration

[sleidig]

Observation from real-life system related to this:
When importing or bulk editing/deleting thousands of records all changes are processed one by one also. This can easily take an hour in practice until the system calms down again.