cache_req: improve debugging
    
    The original debugging was unnecessarily large. I removed request name
    from all debug messages (keeping the request number). The name is now
    print only when a new request is created. I also added an explicit message
    to see what object are we currently looking up.
    
    For example:
    [cache_req_set_plugin] (0x2000): CR #4: Setting "Group by ID" plugin
    [cache_req_send] (0x0400): CR #4: New request 'Group by ID'
    [cache_req_select_domains] (0x0400): CR #4: Performing a multi-domain search
    [cache_req_set_domain] (0x0400): CR #4: Using domain [LDAP.PB]
    [cache_req_search_send] (0x0400): CR #4: Looking up GID:20001@LDAP.PB
    [cache_req_search_ncache] (0x0400): CR #4: Checking negative cache for [GID:20001@LDAP.PB]
    [cache_req_search_ncache] (0x0400): CR #4: [GID:20001@LDAP.PB] is not present in negative cache
    [cache_req_search_cache] (0x0400): CR #4: Looking up [GID:20001@LDAP.PB] in cache
    [cache_req_search_send] (0x0400): CR #4: Returning [GID:20001@LDAP.PB] from cache
    [cache_req_create_and_add_result] (0x0400): CR #4: Found 1 entries in domain LDAP.PB
    [cache_req_done] (0x0400): CR #4: Finished: Success
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>