Better connection management for http host

[incfly]

Currently the http.cc https://github.com/istio-ecosystem/authservice/blob/master/src/common/http/http.cc Get Post methods.

Will initiate one connection for every request.
The actual usage typically has many requests per host.
We are also waiting connection to be teared down correctly before returning the http response.

A first step (subtask) could be changing the way we close the connection, use async_wait plus time out.

We should consider the better connection reuse for http requests.