WebAssembly Strategy/`Send` bounds

[insipx]

Problem Description

Make it possible to use diesel_async from a WebAssembly context. Often, WebAssembly code needs to forgo the Send requirement in order to interact with asynchronous code. more context: #174

What are you trying to accomplish?

Using an async diesel backend in WebAssembly

Checklist

• I have already looked over the issue tracker for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate