ConfdClient.SendRequest: allow max coverage

This patch changes the coverage parameter to allow specification of max
coverage (via -1), versus auto-computation (default, 0) and manual
specification.

Unittests are updated for this case too.

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