Make structures Send

[bltavares]

Choose one: a 🙋 feature

Using hypercore on a multi-threaded async/await executor requires it to be send, requiring this stream also to be send.

This commit adds an example which requires the crate to be send, and changes all references to Rc with Arc.

Checklist

• tests pass
• tests and/or benchmarks are included: async example
• documentation is changed or added

Context

• example to ensure structs are send hypercore#95
• Implement peers / network hypercore#11 (comment)

Semver Changes

Major, as it breaks impls of the trait

[yoshuawuyts]

Excellent!