Optimise intchron()

[joeroe]

We should try to make IntChron queries as fast as possible. Currently, retrieving the entire database (without tabulation) takes:

system.time(intchron("all", tabulate = FALSE))
#>   user  system elapsed 
#> 48.587   0.579 436.211 

user+system time is low (but could still be improved), so we can assume most of the time is spent waiting for responses to IntChron requests.

Ideas for optimisation:

• Group batches of requests into vectorised calls to intchron_request() as often as possible
• Parallelise requests:
  • At the least it should be safe to execute recursive calls to intchron_crawl() in parallel, since crawling each branch is independent of other parts of the tree.
  • Also investigate using ... in intchron_request() and possibly elsewhere.
  • But need to ensure these don't place too much load on the IntChron server, since we already get timeouts when running the relatively modest queries in the examples and vignettes simultaneously as R CMD checks.
• Improve filtering/pattern-matching options in intchron_crawl() and use them aggressively in intchron()
• Investigate where the user CPU time is being spent