Redis backend: pipeline and batching improvements

[allen0099]

Redis backend pipeline and batching improvements. Not breaking.

Split into one sub-issue per pull request:

• Redis get_cache_data: use a non-transactional, chunked pipeline #171
• Redis clear operations: delete page by page and drop the extra EXISTS #172
• Redis get_all_keys: deduplicate keys returned twice by SCAN #173

[allen0099]

All sub-issues are done; closing the tracking issue.