Add Confd client to the Haskell code base

The client queries all the master candidates in parallel, until the minimum
number of replies, defined in the constant file, is received.
A timeout prevents the waiting from being of indefinite length.

The reply to be returned to the function that made the query is decided
according to the Confd design document.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>