work around multihreading issue

[maxheld83]

there appears #158 to be an issue with multithreading, where several API requests by the same machine (same IP) appear to lock one another.
Potentially because of issues with the handle #158.

If that can be reprexed, the solutions are, in order of preference:

1. use shiny promises to queue the render jobs in the background, so there's always only one of them working at any one time.
   (If that is possible, I am not sure)
   To clarify, the issue here is not whether the Alice's shiny session is blocked by Bob, or by Alice, but whether a job started by Alice can wait for all other jobs to be finished.
2. hack around this limitation with purrr and exp backoff.
3. Call crossref directly with httr, though it would need to be reprexed whether this is an issue.

[maxheld83]

would be obviated by #233