Related:
    https://fedorahosted.org/sssd/ticket/2553

Adds two new functions to the cache_req API:
    - cache_req_user_by_filter_send
    - cache_req_group_by_filter_send

These functions can be used to retrieve users or groups that match a
specified filter.

Also renames a variable to avoid constant confusion -- the variable is
only used for debug output.

Reviewed-by: Pavel Březina <pbrezina@redhat.com>

Related to https://fedorahosted.org/sssd/ticket/2596

Reviewed-by: Pavel Březina <pbrezina@redhat.com>

The input name is now parse automatically by cache_req if
none particullar domain is specified. The parsed named is
returned from _recv as an output parameter.

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Currently cache_req takes only user name as an input parameter. However,
this is not enough since we will need also UID and GID in the future.
This patch creates a structure to hold input parameters so it can be
simply extended to support other input types.

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Many areas of responders performs an expiration check and refresh
of cached objects during single or multiple domain search. This code
is duplicated on many areas of the code with small or none
modifications.

This interface aims to reduce code duplication between responders,
by providing one universal API for requesting cached objects.

This API will take care of cache lookup, expiration check, cache
refresh, out of band cache request, negative cache in both single
and multi domain searches.

Reviewed-by: Michal Židek <mzidek@redhat.com>