Implement caching layer

[calvinnhieu]

Design doc: https://docs.google.com/document/d/1P6boQoVya126qm0JI7oqyJ24bLdUw1cD7-BrGlxPuUs/edit

Changes:

• Generate and store RequestHash field in Dynamo RequestTable
• Implement cache hit path in query_runner.py
• Update output matrix S3 prefix to include data_version and request_hash
• Update GET endpoint to retrieve cached entries

DoD:

• Requesting the same matrix multiple times (defined by resulting from the same cellkey set) does not generate a new matrix and returns a copy of an existing matrix
• No regression in v0

[brianraymor]

This was postponed to Milestone 2. We need to buffer a bit more time for emerging issues such as https://humancellatlas.slack.com/archives/CC9AR11T7/p1563988186044800