worker: Allow runner to go back to LOADING post-start

[victorges]

What does this pull request do? Explain your changes. (required)
This is a change required by livepeer/ai-runner#798
so that the runner containers are "allowed" to go back to the LOADING state
after startup. This will be used to avoid sending streams to a container that
is still returning to the default params after handling a stream.

Specific updates (required)

• Reset the loadingStartTime every time we see the LOADING state
• thats all lol

How did you test each of these updates (required)
Ran the box and tested multiple loading scenarios, during stream, after stream, etc.

Does this pull request close any open issues?
Part of INF-297

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[linear]

INF-297 Propagate loading state

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.94865%. Comparing base (d095d96) to head (78fd117).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3743         +/-   ##
===================================================
+ Coverage   31.94301%   31.94865%   +0.00564%     
===================================================
  Files            158         158                 
  Lines          47519       47520          +1     
===================================================
+ Hits           15179       15182          +3     
+ Misses         31437       31436          -1     
+ Partials         903         902          -1     

Files with missing lines	Coverage Δ
ai/worker/docker.go	70.76649% <100.00000%> (+0.23078%)	⬆️

... and 2 files with indirect coverage changes

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Files with missing lines	Coverage Δ
ai/worker/docker.go	70.76649% <100.00000%> (+0.23078%)	⬆️

... and 2 files with indirect coverage changes

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[j0sh]

LGTM