Stasher Release Candidate Build

This release introduces the Stasher Caching Repo to Drafter,

Issues included are:

• Original Stasher implementation
• #221 Disk Space cleaner. Removing expired, inaccessible, or just not frequently enough used cache entries
• #252 Caching of State Graph Queries. In particular the modified times that we use internally in Stasher.

Each contains their own sub-issues, dive deeper for more information.

Other important mentions:

• #246, #265 Datadog metrics for Stasher, capturing times and counts for database/cache queries (hits and misses), and web requests into Drafter as well
• #267 Stasher configuration (environment variables)