Update examples to async/await

[weihanglo]

What

#1849

Update following examples to async/await version and move them from examples_disabled to examples folder.

• proxy 152af75
• state 9002bac
• single_threaded 3984682
• upgrade c353d98

How to test

proxy

This is a example for a proxy server. It would proxy every requests from localhost:3000 to localhost:3001. To test its functionalities, first

• Launch the proxy server by running cargo run --example proxy
• Launch the target server by running cargo run --example hello
• Run curl localhost:3001 and response Hello World!

single_threaded & state

These two examples are counterparts of each other. Only in singled_threaded example can Rc (not Send type) be used.

• Run cargo run --example state to launch the server.
• curl localhost:3000 and check if the request counter increases correctly.
• Run cargo run --example singled_threaded to launch the server.
• curl localhost:3000 and check if the request counter increases correctly.

upgrades

This example demos HTTP upgrading. To test it, just follow the steps:

• Run cargo run --example upgrades to launch the server.
• The console would output followings

  client[foobar] sent
  server[foobar] recv: Ok("foo=bar")
  server[foobar] sent
  client[foobar] recv: Ok("barr=foo")
  

• The server then shutdowns gracefully.