Relicense the crate under MIT/Apache 2.0

[weiznich]

This issue tracks the progress to relicense diesel_async from AGPL 3.0 to MIT/Apache 2.0.

I merged contributions from the following people:

• @smklein
  • Fix documentation links #8
• @Razican
  • Using Diesel 2.0 rc1 #15
• @gwy15
  • fix typos #21
• @fluxxu
  • Fix the type alias for deadpool's BuildError #30 (already consent to transfer the ownership of this change to me)
• @johnchildren
  • Re-export RunError and PooledConnection from bb8 #35 (already consent to transfer the ownership of this change to me)
• @livingcorpse25
  • Pooled connection (mobc) documentation improvement #37 (already consent to transfer the ownership of this change to me)
• @treysidechain
  • add usage of scoped futures to transactions: ownership of this change transferred to @weiznich #38 (already consent to transfer the ownership of this change to me)
• @medcat
  • Re-export of TransactionManagerStatus #40 (already consent to transfer the ownership of this change to me)

For @smklein, @Razican and @gwy15: Please comment on this issue whether you agree to relicense your code or not?
For @fluxxu, @johnchildren, @livingcorpse25, @treysidechain: I already checked your boxes above, as you agreed to transfer the ownership of your changes to me. Please comment if you disagree with this change, otherwise I assume your consent.

[smklein]

I agree to relicense (and I'll check my own box). Thank you for doing this!

EDIT: Okay, I can't check my own box, but I consent for you to do it.

[Razican]

I also agree to relicense the code :)