Performance addendum for filters PR
Shipping methods are calcualted whether or not all of the information is available for the calcualtion.
This can take a long time, expecially when shipping methods require some minimum set of information.  The
filter added allows a theme, plugin or or a shipping module to prevent the time consuming calulation and
perhaps even record record an error or present an informitive message.

When combined with the new customer profile infrastructre this can have a positive impact on performance and
cehckout flow.