Fixes an issue in backend probe initialization that causes the default
value of .initial to equal .threshold.

The default for .initial is now .threshold - 1, meaning that the backend
starts up as sick and a single successful poll is required for it to be
considered healthy.

With the previous behaviour, a sick backend would not be considered sick
until after the initial polls exit the polling window.

Thanks to Colin Curtin.

Fixes: #1227