[Feature Request] HTTP Pipelining

[romainreignier]

Currently, Pistache does not support HTTP Pipelining: https://en.wikipedia.org/wiki/HTTP_pipelining

With a client using Qt's QNetworkRequest with Pipelining enabled, I am missing several requests.

From the code, the buffer is clear once the first request is parsed, losing the other requests in the same buffer: https://github.com/pistacheio/pistache/blob/e619e7e90bb313df196cde55db978e843d5d2cfe/src/common/http.cc#L1107