asynchronous requests to CATMAID API

[jefferis]

We could think about support for async requests e.g. fetching neurons in parallel could be much quicker. The nicest way to do this might be to have something like catmaid_fetch_multi where you could set the number of async requests. You might then need to do some work to join results up. Would also need to think about how to specify separate lists of POST body information (maybe GET support would cover a lot of use cases initially).

In terms of package support

• httr NO Possible to implement asynchronous (i.e. non-blocking io) in httr? r-lib/httr#271
• crul YES see https://ropensci.github.io/crul/reference/Async.html
• curl YES but perhaps a bit more complex https://cran.r-project.org/web/packages/curl/vignettes/intro.html#async_requests
• Also this new package: https://github.com/r-lib/async but not on CRAN yet.