Drop session_path column from datasets cache table

[k1o0]

Dropping the redundant session_path column from the datasets cache table and using joins instead we would reduce the table size in memory by about 13%-23% for a (hopefully) negligible increase in time loading datasets.

• Drop session_path column in cache loader
• Method to get session path from Series (ab40955)
• Remove session_path from standard column list and local cache generator functions
• Handle missing session behaviour when loading aggregate datasets
• Update converter methods to handle missing session path column (i.e. record2url, path2url)
• Update download_datasets and download_aws to handle session path lookups