Log requests and request failures in build daemon asset server

[annagrin]

• Add --log-requests flag to build runner daemon command.
• Log request failures in asset server.
• Configure http client in tests to limit max connections per host,
  to prevent CI from failing with reset connection errors.

Extra logging will help diagnose CI flakes in build:

https://github.com/dart-lang/build/runs/3062926301

and webdev:

Related: dart-lang/webdev#1345

[annagrin]

@natebosch @grouma @jakemac53 I am thinking about cancelling this PR, as I actually only need one change in it (logging the exception that happens in the asset server handler). I realized that I can log requests from webdev as well, without adding flags to the build daemon, and the rest of the changes are just to enable more logging to debug the failures I encountered in this PR.

Anyway, it helped me narrow down the actual issue and find the solution for it so there was a benefit:) I'll submit a new one just for logging exceptions.

Update: would be still good to keep the tests. I will remove all the parts I don't need and keep http config change, tests, and logging exception in asset server handler.

Update: I decided to just modify this PR instead of cancelling. I kept the --log-requests flag to the daemon as well as the changes mentioned above, it can be useful for debugging the requests to the server.

[annagrin]

@natebosch @jakemac53 @grouma I addressed (i think) all comments.

Changes I kept - let me know if you would like me to remove any of them:

• add a --log-requests flag to build daemon
• test parallel download of many files in _test/test/serve_integration_tests.dart
• test parallel download of many files in build_runner/test/serve_integration_tests.dart
• log failures in asset server handler
• limit simultaneous connections on http client in tests to prevent flakes in CI.