Auto merge of #2154 - jtgeibel:prod/reduce-dl-count-contention, r=sgrif
Auto merge of #2154 - jtgeibel:prod/reduce-dl-count-contention, r=sgrif

Update `version_downloads.counted` last to minimize lock contention

If we defer the update to `version_downloads.counted` until immediately
before the commit, it should eliminate the contention between counting
new downloads and propagating previous downloads.

Refs: #2153
See also: https://github.com/rust-lang/crates.io/issues/2153#issuecomment-577504032

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