R1:b479941ceb6e

Make "Range" HTTP header work for Celerity static resource requests

Summary:
Ref T7567. In T8266 I fixed a bunch of obscure "Range" issues, but only for file downloads -- not for Celerity.

Extend all that stuff to Celerity, which is fortunately much easier.

I believe this will fix Conpherence sounds in Safari.

Test Plan:
- Wrote out an HTTP request in a text file with `Range: bytes=0-1` and similar, piped it to localhost with `cat request.txt | nc localhost 80`, saw server return appropriate range…