Implement simple LDAP cache layer
    
    Insert a class before LDAPClient to cache the return value
    of get_entry() and certain exceptions (NotFound and
    EmptyResult). The cache uses an OrderedDict for the cases
    where a large cache might result an LRU model can be used.
    
    The cache be enabled (default) or disabled using
    ldap_cache=True/False.
    
    This cache is per-request so is not expected to grow
    particularly large except in the case of a large batch
    command.
    
    The key to the cache entry is the dn of the object
    being requested.
    
    Any write to or referencing a cached dn is evicted from
    the cache.
    
    The set of attributes is somewhat taken into consideration.
    "*" does not always match everything being asked for by
    a plugin so unless the requested set of attributes is a
    direct subset of what is cached it will be re-fetched. Err
    on the side of safety.
    
    Despite this rather conserative approach to caching 29%
    of queries are saved with ipatests/xmlrpc_tests/*
    
    https://pagure.io/freeipa/issue/8798
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Rafael Guterres Jeffman <rjeffman@redhat.com>