Make dns_dt_send() call dns_dt_reopen() asynchronously

Make dns_dt_send() call dns_dt_reopen() asynchronously

Instead of checking current dnstap output file size and potentially
synchronously calling dns_dt_reopen() upon every call to dns_dt_send():

  - call dns_dt_reopen() asynchronously by queuing an event to the task
    specified at dnstap environment creation time,

  - ensure no roll event is outstanding before checking dnstap output
    file size and potentially queuing another roll event.

This causes dnstap output files to exceed their configured size limits,
but prevents any two threads from performing the roll simultaneously
(which causes crashes).