4411. [func] "rndc dnstap -roll" automatically rolls the
dnstap output file; the previous version is
saved with ".0" suffix, and earlier versions
with ".1" and so on. An optional numeric argument
indicates how many prior files to save. [RT #42830]
Added API to create a set of UDP dispatches which can be shared
round-robin style when making upstream queries for authoritative
data; this should reduce lock contention in the query source
dispatch.