Add a new confd callback (StoreResultCallback)
Add a new confd callback (StoreResultCallback)

This new callback simply stores (without calling any lower-level
callback) the last result; coupled with the filtering callback, this
ensures that it has the 'best' response after all have been received.

The result can then be retrieved via the GetResponse method.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,andir/ganeti,ganeti/ganeti,bitemyapp/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,apyrgio/snf-ganeti,andir/ganeti,leshchevds/ganeti,mbakke/ganeti,bitemyapp/ganeti,ganeti/ganeti,apyrgio/ganeti,onponomarev/ganeti,onponomarev/ganeti,dimara/ganeti,mbakke/ganeti,grnet/snf-ganeti,leshchevds/ganeti,dimara/ganeti,yiannist/ganeti,yiannist/ganeti,andir/ganeti,mbakke/ganeti,yiannist/ganeti,apyrgio/ganeti,apyrgio/snf-ganeti