test concurrent runs

[maxheld83]

we really need async renders asap #42 now.

This was so far blocked, because we need all code, and especially side effects and outbound calls to live happily concurrently on one instance, sharing the same IP address, disc and everything else.

This has bitten us a couple of times, probably including:

• race condition for reading csvs read_csv in apc getter causes race condition #86
• race condition for sending emails talk to smtp relay via rest api #98 get status from smtp send #101 (?)
• race condition querying github fix github open apc auth bug #56

I say "probably" because on live shiny apps it's very hard to test this, because two requests would have to come at the exact same time if run async, and that can make the bug appear intermittently

There's also the issue of whether this works with cr tracked separately in #135.

Before getting the async shiny in #42 to work (which isn't hard), we should test whether we have to expect/fix any concurrency issues, so that doesn't bite us again.