We keep various information bits about the server around, some only
effecting the currently handled file (like sizes) while others
should be persistent (like pipeline detections). http used to reset all
file-related manually, which is a bit silly if we already have a Reset()
method – which does reset all through –, so extending it with a
parameter for reuse and calling it from https too (as this was
previously resetting by just creating a new state struct – it uses no
value of the persistent state-keeping yet as it supports no pipelining).