In WT-7076 the infrastructure of the internal worker thread for tiered storage was added but that thread does not do anything. This ticket is to implement the work of the internal thread. It will include:

• Creation of tiered work elements that indicate an operation to perform and the name to perform it on.
• Copying the object using the storage source to the shared location.
• Updating the object's metadata to set flush to true. (Created in WT-7497)
• Call storage source migrate method to move to a cached location. (this item may be split out if other infrastructure does not yet exist for this.)