Make it possible to just reset the internal cache of node_load()

node_load() has a $reset parameter, which allows to not increase the used memory
in case you have to iterate over a good couple of nodes.

This works fine on a ordinary drupal site, but has problems
once proper (non-static) caching is taken into account.

Every call with $reset = TRUE for example triggers a $memcache->delete() call.


* Introduce drupal_static()/drupal_static_reset() from d7.
* Use those components to be able to reset the static cache externally.
* using $reset will work the same in the future, BUT, you can skip $reset
and just call node_load_reset() in case you iterate, which will not call out to the
proper caching.