Ensure pending merges are updated on segment flushes

    
    Due to the default of `async_merge` to `true` we never run
    the merge policy on a segment flush which prevented the
    pending merges from being updated and that caused actual
    pending merges not to contribute to the merge decision.
    
    This commit removes the `index.async.merge` setting. The setting is 
    actually misleading since we take care of this on a different level 
    (the merge scheduler) since 1.1.

    This commit also adds an additional check when to run a refresh
    since solely relying on the dirty flag might leave merges un-refreshed
    which can cause search slowdowns and higher memory consumption.
    
 
Closes #5779