[config] config options to stream request/response (#949, #376)

[config] config options to stream request/response (#949, #376)

This allows admin to configure if response is collected in entirety
prior to sending data to client

For compatibility with existing configs, default is existing behavior:
  buffer entire response prior to sending data to client

The following are config options, though not all implemented yet

// default: buffer entire request body before connecting to backend
server.stream-request-body = 0

// stream request body to backend; buffer to temp files
server.stream-request-body = 1

// stream request body to backend; minimal buffering might block upload
server.stream-request-body = 2

// default: buffer entire response body before sending to client
server.stream-request-body = 0

// stream response body to client; buffer to temp files
server.stream-request-body = 1

// stream response body to client; minimal buffering might block backend
server.stream-request-body = 2

x-ref:
  "fastcgi, cgi, flush, php5 problem."
  https://redmine.lighttpd.net/issues/949
 "Reimplement upload (POST) handling to match apache/zeus/thttpd/boa functionality"
  https://redmine.lighttpd.net/issues/376