Replace log and env_logger with tracing crates

[Turbo87]

This brings us closer to being able to use structured logging and properly instrumenting some of functions. The change is mostly a drop-in replacement as described by https://docs.rs/tracing/0.1.22/tracing/#for-log-users.

The only difference is the slightly changed log output format:

# before
[2020-12-23T21:51:50Z INFO  cargo_registry::app] test23

# after
Dec 23 22:53:48.847  INFO cargo_registry::app: test23

This could be massaged by using a custom FormatEvent implementation though, if we really need this to be formatted in a specific way. Otherwise there is also a JSON-based output format which might be beneficial in production for real structured logging.

r? @pietroalbini

[pietroalbini]

I think you also need to update the test suite, after #3130 landed.

[Turbo87]

yep, you're totally right 👍

should be fixed now :)

[pietroalbini]

@bors r+

[bors]

📌 Commit 8b36e01 has been approved by pietroalbini

[bors]

⌛ Testing commit 8b36e01 with merge b6bc89f...

[bors]

☀️ Test successful - checks-actions
Approved by: pietroalbini
Pushing b6bc89f to master...