Previously it was sufficient to use the old parser result for a Query if
the DQL didn't change (Query::STATE_CLEAN), but now there must also be
no changes to the filter collection of the EntityManager.
In the old situation each Query object would create a hash of all the
filter objects on it's own. That was not very efficient.
This commit adds the state of the current filter collection to the
EntityManager. The state will be set to FILTERS_STATE_DIRTY as a filter
is enabled, removed or a parameter is set on a filter. The hash is also
computed by the EntityManager, but only if the filter collection is
dirty. This will prevent recalculation of the hash with each query.
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