Pin tokio version to ~1.14

[notmandatory]

Description

The tokio project recently changed their MSRV to 1.49.0. This PR will pin the tokio dependency version to ~1.14 which is prior to their MSRV increase.

Notes to the reviewers

The LDK team took this approach for their tokio dev-dependency, see lightningdevkit/rust-lightning#1315.

As long as tokio backports bug fixes to1.14.x releases this should be safe. If we need any new features we can revisit this decision.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've updated CHANGELOG.md

[notmandatory]

I think we can use this as a short term fix while we discuss #331 to determine what our MSRV should be.

[LLFourn]

ACK b04bb59

I agree this is the right thing to do for now.