New upstream release.

Performance improvements:

• Pkgset phase will reuse entire package set if it was not modified from previous compose.
• Gather phase can also reuse results from previous compose.
• Better parallelization of small createrepo_c processes.
• Extra files phase no longer waits for gathering.

Minor fixes:

• Options for customizing latest symlink.
• Resend message while waiting for ostree ref to be created.
• Config validation can accept overrides for schema, making the validation more strict.