Implement ephemeral lifecycle for Flask v1

[rekmarks]

TODO

• Add configurable Snap request processing time limit #58
• Monitor snap busy and idle time #62
• Terminate idle and/or misbehaving snaps #63
• Add ability to enable / disable snaps #113

Future TODO

• Set limit for number of concurrent snaps #59
• Add Snap request processing queue #117

Specification

• Under normal circumstances, MetaMask will never terminate a snap while it's processing a request, before the processing time limit has expired.
  • The processing time in v1 of the MVP will be on the order of 30-60 seconds.
• MetaMask may terminate a snap immediately after the processing time limit has expired, or once processing has completed, which we define as when MetaMask observes the snap sending a response to a particular request.
• Under abnormal circumstances, MetaMask may be forced to terminate a snap while it's processing a request, but will only do so if absolutely necessary.
  • Such circumstances include:
    • If the snap is consuming too many resources, where the threshold for "too many" may vary with the number of snaps running concurrently, and other factors.
    • If the snap "hangs" and its worker becomes unresponsive.
    • If the user force-quits the snap.
• Regardless of when and how a snap is terminated, it should gracefully start back up.
  • If a snap's state becomes so corrupted that it cannot be restarted, the user may have limited options for resolving the matter, short of reinstalling the snap.

[rekmarks]

This was done.