Release plan for support on tokio v0.3?

[LEXUGE]

I see tokio v0.3 is supported on git version, however, there seems like no plan date available to release, which makes some downstream crates hard to update (https://github.com/bluejekyll/trust-dns/pull/1262#issue-509465856).

Do we have a date on release? Or can we have an alpha version being released early?

[bluejekyll]

I'd like to suggest that perhaps the hesitance on releasing h2 in the face of possibly needing other breaking changes for hyper or other downstream dependencies, is probably being a little overly conservative. This library is generally not a user facing library. Most folks will be building with other libraries as the primary usage of this, giving it a bit of an air-gap from breaking changes effecting a significant number of projects, see reverse_dependencies.

I don't want to speak for others, but I think it would be a small amount of work to update any of those libraries even if a 0.4 needs to be released due to issues integrating with hyper or other downstream libraries.

[taiki-e]

I think we can close this as #504 merged and 0.3.0 released.