Fix locking bug causing high CPU usage

Iustin Pop noticed unusually high CPU usage with 2.1's master
daemon, even with very simple opcodes like OP_TEST_DELAY. As
it turns out, we inadvertently passed seconds as milliseconds
to a call to poll(2). Due to the way the loop around the call
works it didn't break competely, but caused higher CPU usage
by the poll(2) call returning too early.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>