loadtest.c: when deleting, always make sure we deleted 1 message.

loadtest.c: when deleting, always make sure we deleted 1 message.

If there are multiple loadtest processes running -- and that's really
the whole idea -- we will often try to delete a message that was already
deleted by another process.  That's not an error condition, but it would
cause posts to outnumber deletes and the rooms would grow endlessly.  So
if we didn't successfully delete something, we go back and try deleting
another one.

Fun fact: the algorithm used to decide which message to delete is the same
algorithm used in the 'fortune' program.