Werk #11938: REST API: make optimistic locking via ETags configurable

When multiple HTTP clients want to update an object at the same time, it can happen that the slower client will overwrite changes by the faster one. This is commonly referred to as the 'lost update problem'. To prevent this situation from happening, Checkmk's REST API does 'optimistic locking' using HTTP ETag headers. In this case the object's ETag has to be sent to the server with an HTTP If-Match header, for the object to be modified.

For simplified usage in small installations this behavior can now be deactivated, but this also deactivates the 'lost update problem' prevention. In that case there should only be one script which sends updates via the REST API running at a time, to prevent data from getting lost.

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