Support asyncio

[xli]

Async client has significant better performance and scalability.
It will become harder and harder to switch to async IO when more features are built.
We may keep current blocking client using requests package for backward compatible.

• async jsonrpc client and faucet client
• async miniwallet app
• async miniwallet client and testing suites
• dmw cli support async
• update example code to use async clients, or remove examples that are not useful
• update doc and refer to mini-wallet app for some examples.
• Delete offchain http_server.py, which is a blocking server