Panic `executor must be set` occurs when hyper dep with "tcp" feat enabled is not in dependencies

[threema-donat]

Steps to reproduce:

• Create project without the same hyper version as dependency, e.g. with hyper 1
• Add a2 as dep
• Push to APNS

--> panic executor must be set

I don't know why this was removed again in this commit, but it seems like a mistake.

[threema-donat]

See https://github.com/WalletConnect/a2/issues/59 for reference