[SSSD] [PATCHES] cache_req improvements

[Pavel Březina]

Hi,
these patches were created as part of my dbus work however they do not 
depend on it. Those patches can be applied on top of current master and 
review separately.

Recently introduced cache_req interface which hides all the logic of 
search for a sysdb object in cache and data provider supported only user 
by name and initgroups request.

These patches add support also for "user by id", "group by name",
"group 
by id" (for new Users and Groups IFP interfaces). It also hides 
sss_parse_inp into cache_req so the may no longer be the need to parse 
username in consumers of the API.

Everything is unit tested.

Happy reviewing :-)