light-client: follow-up

[romac]

• Introduce clockDrift parameter, as per the spec
• Write documentation
• Tag code as per the traceability proposal
• Add new tags to the English spec
• Add tag versions to the English spec
• See whether we need to re-introduce the TrustedStoreReader/Writer wrappers (we don't)
• Add support for verifying headers lower than the latest trusted state - Implement minimal support for backward verification in light client #361
• Add structured logging
• Streamline creation of a light client instance and supervisor
  • Use the builder pattern to create and initialize the LightClient and Supervisor
• Write units tests for new components - [light-client] Testing Regimen meta-issue #320
• Port current light client unit tests - [light-client] Testing Regimen meta-issue #320
• Remove previous lite client code in tendermint crate - Deduplicate Light Client code #342
• Address comments in Light Client Refactor Review #294

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Other issues we may want to fix as well as part of this follow-up:

• Light client testing: remove implementation specific assertions about number of bisections #276
• Light client time followup #137
• light-client: consider optimizing voting power calculation by stopping after verifying +1/3 or +2/3  #235
• Light Client minor improvements #138
• Light client crate structure #136 (mostly done in Light Client refactoring #237)

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These tasks should be handled in separate issue and PRs:

• Implement CLI for running a standalone light client + RPC server

[ebuchman]

I believe all the points have either been addressed or given new issues except for:

• Add structured logging
• Streamline creation of a light client instance
  • Use the builder pattern to create and initialize the LightClient

@romac want to open separate issues for these and then we can close this one?

[romac]

Done in #421 and #422.