Do not count downloads as in-flight
Do not count downloads as in-flight

Download requests are no longer counted as in-flight and the variable
name is updated to reflect this. By default 20% of database connections
are still reserved for updating download counts, but other requests will
be rejected less often. Additionally this saves 2 atomic updates when
serving the most popular request.

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io