[json-rpc-engine] Refactor middleware to be async by default, deprecate `createAsyncMiddleware`

[MajorLift]

Prior to the @metamask/json-rpc-engine migration into the core monorepo, there was an ongoing initiative spearheaded by @rekmarks to simplify json-rpc-engine middleware, and make them async by default:

• Allow middleware to be async, deprecate createAsyncMiddleware json-rpc-engine#81
• RFC: Remove callback from next return handlers json-rpc-engine#82
• RFC: Remove next callbacks json-rpc-engine#83
• WIP: Improve middleware errors json-rpc-engine#84
• Add DuplexJsonRpcEngine json-rpc-engine#107

The team intends to follow-up on this to bring these efforts to completion.

[Gudahtt]

Here's another branch where I attempted to rewrite JSON-RPC engine with similar goals in mind: https://github.com/MetaMask/json-rpc-engine/tree/rewrite

@rekmarks and I were unaware of each others work at the time, but reached very similar conclusions in the end I think. His is probably a better starting point for picking this back up because it's already split up. I'm linking mine mostly for my own reference.