Feature/internal api url

[timseidel]

Free Speed improvement by skipping DNS, TLS and Proxy when using the formr API in openCPU. Fallback is current behaviour.

Quick Benchmark for a api call (ca 30% faster):

Code:

formr_api_authenticate()

n <- 100
elapsed <- numeric(n)
for (i in seq_len(n)) {
  t0 <- Sys.time()
  s <- formr_api_sessions(.formr$run_name, limit = 1)
  elapsed[i] <- as.numeric(difftime(Sys.time(), t0, units = "secs"))
}

cat(sprintf("Host: %s\n", .formr$host))
cat(sprintf("formr_api_sessions (n=%d):\n", n))
cat(sprintf("  avg: %.1f ms\n", mean(elapsed)*1000))
cat(sprintf("  min: %.1f ms\n", min(elapsed)*1000))
cat(sprintf("  max: %.1f ms\n", max(elapsed)*1000))
cat(sprintf("  p50: %.1f ms\n", median(elapsed)*1000))

Host: https://formr.rstudy.org/api
formr_api_sessions (n=100):
avg: 63.9 ms
min: 43.1 ms
max: 176.1 ms
p50: 58.9 ms

Host: http://formr_app/api
formr_api_sessions (n=100):
avg: 43.8 ms
min: 25.2 ms
max: 163.2 ms
p50: 38.5 ms