cache: Add new nl_cache_find api

This patch adds new cache find api

nl_cache_find api was suggested by Thomas.

Unlike nl_cache_search, this patch uses
nl_object_match_filter() to look for an
object match.

Am not sure this matches what was decided
on the list few weeks back. I will be happy
to make any changes.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>