Currently, there is no way for a user of the confd client library to
know how many replies there should be, whether all have been received,
etc. This is bad since we can't reliably detect the consistency of the
results.

This patch attempts to fix this by adding a synchronous WaitForReply
function that will wait until either a timeout expires, or until a
minimum number of replies have been received (interested users should
add similar functionality for the async case). The callback
functionality will still do call-backs into the user-provided code
during the wait, but after this function has returned, we know that we
received all possible replies.

Note: To account for the interval between initial send of the request,
and calling of this function, we modify the expiration time of the
request.

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