Prior to this change, if a CachingDescriptorSystem was prepopulated
with XBlock field data (`module_data`), then it would be used as
the DescriptorSystem for blocks loaded through it. On the other hand,
if it had to fetch new data from the modulestore, then a new
CachingDescriptorSystem would be created by the modulestore, and used
by the newly returned object.
This commit allows the modulestore get_item and get_items methods
to accept an existing CachingDescriptorSystem to augment with data
from the backing mongo store, and to use as the DescriptorSystem
for newly loaded XBlocks.
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