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 sets the `async_merge` to `false` by default to enforce
    updateing the pending merges. 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 soely relying on the dirty flag might leave merges un-refreshed
    which can cause search slowdowns and higher memory consumption.
    
    Closes #5779