Improve ETag performance

We should improve the ETag performance.

# Node ETag

The ETag gets computed by checking the following values/references:

* branch -> UUID
* parent node (for branch) -> UUID [**node events**]
* container (for lang, branch, version) -> container ETag [**node events**]
* various query params (resolve links, expanded fields)
* Tags of the node -> Tag ETags [**node tag events**]
* Node children -> amount, schema name [**node events**]
* container lang -> Lang [**node events**]
* breadcrumb [**node events**]
* resolved paths [**node, project, branch events**] (due to branch hostname)
* role permissions [**role, user, group events**]

# Option 1 - ETag Cache

* Implement a global ETag cache which returns the value if present
* Invalidate the cache by registering on the eventbus. Events which indicate that element have been updated could invalidate the ETag cache.
* Check whether it would be possible to generate a list of element that had to be checked to generate the ETag. Invalidate the cache entry if any of the listed elements for the ETag gets modified.