Optimize blockstore cache
Optimize blockstore cache

First off, this fixes a bug where BundleCache would only set caches for
the duration of the default timeout, defeating the cache versioning
strategy.

Second, this adds an optimization so that bundle draft files are cached
as soon as possible.

angelapper/edx-platform,edx/edx-platform,edx/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,edx/edx-platform,msegado/edx-platform,EDUlib/edx-platform,msegado/edx-platform,eduNEXT/edx-platform,stvstnfrd/edx-platform,arbrandes/edx-platform,edx/edx-platform,eduNEXT/edx-platform,eduNEXT/edunext-platform,stvstnfrd/edx-platform,stvstnfrd/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,EDUlib/edx-platform,msegado/edx-platform,msegado/edx-platform,angelapper/edx-platform,msegado/edx-platform,EDUlib/edx-platform,stvstnfrd/edx-platform,eduNEXT/edx-platform,EDUlib/edx-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,eduNEXT/edx-platform,arbrandes/edx-platform