Optimize prefetch function
Optimize prefetch function

Previously the prefetch function would have made a lot of postconf
calls, since it called fetch_resources for every resource in the
catalog. This commit limits the calls to every distinct confdir, which
can have a large impact on the runtime (in case there are lot of resources
of that type). Thanks to Raphaël Pinson who brought me on this and helped
with an efficient implementation.

aptituz/puppet-postfix,aptituz/puppet-postfix