Chunked: Unit can send data to target from buffers before full stream is received

[avahahn]

Unit currently buffers the entire client request before sending it to an upstream target. This target could be an application or an upstream HTTP service. Either way, Unit will not start transferring data until it has the whole request from the client.

In order to support more expedient request processing Unit could possibly start streaming its requests to upstream and application targets as soon as it begins receiving data from the client. In order to accomplish that this ticket embodies the following work

• Identifying features in request processing that need to act on the request data before it gets to the upstream (rewrites, authentication, etc).
• Identifying which interfaces need to be extended to support operating on an incoming stream of data and extending them.
• Modifying the Unit event engine to start sending data to targets immediately as incoming data from the client is received and processed.