Facet expiration flag
    
    Problem:
    
    For performance reason a facet may cache the data in browser's memory. There should be a flag to indicate whether a facet has expired and should be refreshed. The expired flag could be set by these events:
    
     1) any update operation
     2) changing search filter in search facet
     3) switching page in a multi-paged search/association facet
     4) switching direct/indirect view in association facet
     5) facet expiration time
    A facet should be able to use these methods to refresh itself:
     6) on demand: an expired facet should be refreshed when a user opens it.
     7) automatic: an open facet should automatically refresh itself when it expires.
    
    Solution:
    
    This patch solves cases: #2, #3, #5, #6. Case #4 works without any change. Case #1 will be solved later. Case #7 is deffered.
    
    Default expiration timeout was set to 10 minutes.
    
    In this patch are also updated facet.needs_update methods to reflect changes in containing facets.
    
    https://fedorahosted.org/freeipa/ticket/2075