context_page_build() clears caches too aggressively

If the comments are to believed, the call to drupal_static_reset('context_reaction_block_list'); is needed for overlay form submissions. If we're not dealing with one, no need to clear the cache.

This seems to trigger the behavior described in #1172280: Fusion themes render blocks twice where the Fusion theme ends up triggering all blocks to be built twice because by the time its second call to block_list() comes around the cache has been wiped.

Patch forthcoming.