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.

openedx/core/djangolib/blockstore_cache.py

@@ -81,7 +81,7 @@ class BundleCache(object):
         """
         assert isinstance(key_parts, (list, tuple))
         full_key = _get_versioned_cache_key(self.bundle_uuid, self.draft_name, key_parts)
-        return cache.set(full_key, value)
+        return cache.set(full_key, value, timeout=None)
 
     def clear(self):
         """
@@ -101,6 +101,14 @@ class BundleCache(object):
         cache.delete(cache_key)
 
 
+def _construct_versioned_cache_key(bundle_uuid, version_num, key_parts, draft_name=None):
+    cache_key = str(bundle_uuid)
+    if draft_name:
+        cache_key += ":" + draft_name
+    cache_key += ":" + str(version_num) + ":" + ":".join(key_parts)
+    return cache_key
+
+
 def _get_versioned_cache_key(bundle_uuid, draft_name, key_parts):
     """
     Generate a cache key string that can be used to store data about the current
@@ -112,7 +120,7 @@ def _get_versioned_cache_key(bundle_uuid, draft_name, key_parts):
     """
     assert isinstance(bundle_uuid, UUID)
     version_num = get_bundle_version_number(bundle_uuid, draft_name)
-    return str(bundle_uuid) + ":" + str(version_num) + ":" + ":".join(key_parts)
+    return _construct_versioned_cache_key(bundle_uuid, version_num, key_parts, draft_name)
 
 
 def get_bundle_version_number(bundle_uuid, draft_name=None):
@@ -135,6 +143,11 @@ def get_bundle_version_number(bundle_uuid, draft_name=None):
             # Convert the 'updated_at' datetime info an integer value with microsecond accuracy.
             updated_at_timestamp = (draft_metadata.updated_at - datetime(1970, 1, 1, tzinfo=UTC)).total_seconds()
             version = int(updated_at_timestamp * 1e6)
+            # Cache the draft files using the version.  This saves an API call when the draft is first retrieved.
+            draft_files = list(draft_metadata.files.values())
+            draft_files_cache_key = _construct_versioned_cache_key(
+                bundle_uuid, version, ('bundle_draft_files', ), draft_name)
+            cache.set(draft_files_cache_key, draft_files)
     # If we're not using a draft or the draft does not exist [anymore], fall
     # back to the bundle version, if any versions have been published:
     if version == 0 and bundle_metadata.latest_version: