Fix carp being unable to recover from demotion due to send errors

[marius.h_lden.org]

Summary

When carp becomes demoted due to send errors it's supposed to recover and decrement the demotion counter after a certain number of successful sends. This was not working as the send error counter which is doubling as a flag for demotion was reset on the first successful send without taking its value into account. Due to this the code which would actually decrement the demotion counter would never be run.

Test Plan

Test case attached to bug 260499