Implement log flushing

[nkaretnikov]

Fixes #809.

Description

This pull request:

• makes actions write to log storage incrementally: the output can be seen by users as soon as it's available
• changes the logging format by prefixing each line with the name of the action: makes it easier to see what code is running
• adds locking when appending to logs to avoid a race condition
• moves the body of logged_command to ActionContext.run_command, next to run.

Pull request checklist

• [x] Did you test this change locally?
• [ ] Did you update the documentation (if required)?
• [ ] Did you add/update relevant tests for this change (if required)?

Additional information

How to test

• start an environment build
• go to the logs page for this build
• check that new output appears while an action is running, by refreshing the page quickly
• check that all output is prefixed with the action name

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[nkaretnikov]

@dcmcand @peytondmurray A gentle reminder that this still needs review. Will rebase after it's approved.