Slow down crate publish (#1691)

Crate publish used to be slow due to the verification step that `cargo`
would run, which was basically a full compile of the crate prior to
uploading it to crates.io. Now that the `publisher` tool passes
`--no-verify` to `cargo`, it publishes fast enough to get throttled.

This change completely removes the publishing parallelism, adds a fixed
time between publishing each crate, and increases the publish retry
backoff time.