Security Review of GCP Backend

[russjones]

The new GCP backend introduced in #3014 needs a security review before cutting 4.2.

At the minimum session upload needs to be checked that it doesn't clobber an existing session recording.

[awly]

Notes from review:

• session upload could indeed overwrite a previous upload
• lib/backend/firestore key escaping could lead to data corruption by mapping different teleport keys to the same firestore documentIDs
• some minor bugs like unhandled errors, possible nil pointer dereferences, missing connection/resource cleanup
• some stylistic problems (that aren't bugs)

Sent #3766 to fix all found problems, except for stylistic ones.