dlog: Fix issue where the fast-logging broke backward compat

[LukeShu]

This PR is on top of #34, go merge that first.

#28 changed the Logger interface, breaking any existing implementors of the interface. That's not OK.

So revert that interface (and add a comment about how it's frozen), and add an opt-in OptimizedLogger interface for Jose's optimizations.

[thallgren]

Since I consider the non-optimized logger a design flaw, I'd prefer if this PR wasn't merged and the version instead bumped to 2.0.0. That way the backward compatibility wouldn't be broken for 1.x users and the flaw could still be removed.

[thallgren]

Reverting this will break Telepresence, which I assume is OK, given that we're not using a released version.

[LukeShu]

Oh dang, you guys finally upgraded?

Yeah, it's a bad situation because 1.2.4 has been around so long. Do you break the folks who have upgraded to 1.2.4, or break the folks who are on older versions?

[josecv]

this lgtm from a code perspective, i do kind of agree we might wanna release a 2.0.0 but im okay with unbreaking the 1.x line for now

[LukeShu]

Since I consider the non-optimized logger a design flaw

I don't think I agree. I think keeping the required-interface minimal and letting dlog handle as much complexity as it can is a good thing.

[LukeShu]

I've created a "v2" milestone and an issue within it to revisit the dlog.Logger interface: #35 (and also a few other issues that we'll want to consider if/when we do a v2)

[thallgren]

A 2.0 version would be good for three reasons.

1. It wouldn't require this somewhat cludgy solution with casts everywhere.
2. It would give the 1.2 implementers a chance to simply stay put while at the same time give them an incentive to improve their implementation.
3. It would give Telepresence a version that doesn't break us.

My motivation for calling the old version flawed is that unnecessary string manipulation is expensive and the added complexity is very trivial. In fact, in most cases it's just delegated to another log implementation.