cmd/msrserve: initial drop-in replacement server

This is still quite ugly and only meant to get things going for now, as
a proof of concept. It is usable though, and quite fast.

Signed-off-by: Yonah <contrib@gensokyo.uk>

.gitignore

@@ -6,6 +6,7 @@
 *.dylib
 *.pkg
 /msrfetch
+/msrserve
 
 # Content-addressed media files
 /data