logging: Deprecate and remove the logfmt encoder

[mholt]

The logfmt encoder is a very simple key=val format for logs. Unfortunately, it does not support nested structures, which are common in structured logging, even though it could flatten them into a simple key=val sequence. But it doesn't. It's basically broken.

It was probably a mistake to include this encoder in our standard modules, since it yields errors on many log entries, occluding a lot of information that all other formats display just fine.

I'm going to deprecate this module and eventually remove it from our standard modules before more users start relying on it. However, it will still be available in a separate repo just like any other module.

Using this module will emit a warning-level log that logfmt is going to be removed, with a link to this issue. We will eventually remove it from the code base and move it into another repo. (Edit: decided we will just move it to another repo if there's enough demand for it)

To clarify: the logfmt module will still be available, but not by default, as it is essentially broken. Instead of sweeping the bug under the rug, this will call it out to attention, which is better.