Major back-end rework

[KevinMackenzie]

• Merge insert/update paths (Fixes Merge the "Create" and "Update" paths for circuits #504)
• Parameters to server can be specified as env vars or CLI flags (Fixes Switch to env variables instead of flags for backend #710)
• Add key-based circuit sharing (circuit ID + secret key = access) (Preliminary work for Add Circuit Sharing / Permissions #505)
• Removes implicit 1MB circuit limit in datastore (up to ~30MB) (Fixes Add support for multi-part circuits in GCP Datastore #1157)
• Removes sqlite
• Adds tests for GCP datastore / mem database interfaces
• Adds e2e API tests

Currently targeting master with these changes, but that may change.

[KevinMackenzie]

A thought here: If we kept multiple of these objects in the DB, then we can create a coarse history of the circuit. Not hard to add later on (not a breaking change), but I have to think more if this is sufficient for storing "milestone circuits".

[KevinMackenzie]

Thought about this some more. We still have only one ContentHeader pointing to the "current" version. Some kind of timestamp needs to be added to the first ContentChunk entry of a given ContentID to establish a timeline once ContentHeader no longer points to it.