Fix assets with odd extensions and race condition with the same asset at different paths

[ealmloff]

This fixes two bugs with the asset system:

1. Assets with multiple extensions like foo.tar.gz lost the hash because the second extension with the hash (tar-hash) was replaced instead of pushing a the final extension
2. If the asset system includes two files with an identical content hash, but different source paths they were not deduplicated and one of the threads will fail to write the duplicate asset to the output path

Fixes #4432

[jkelleyrtp]

Works on the docsite 👍