This reverts commit 660d5360.

Fixes issue #9.

Update git.autistici.org/ai3/tools/replds digest to b6e6e3c

See merge request !36

Update git.autistici.org/ai3/tools/replds digest to 2ea02f3

See merge request !35

Update git.autistici.org/ai3/go-common digest to b3aa3fb

See merge request !32

Update git.autistici.org/ai3/tools/replds digest to 6456843

See merge request !29

Update git.autistici.org/ai3/go-common digest to 0630401

See merge request !26

Update golang.org/x/crypto digest to c86fa9a

See merge request !27

Update module github.com/miekg/dns to v1.1.50

See merge request !19

Update git.autistici.org/ai3/go-common digest to 39e01d3

See merge request !23

Update module github.com/prometheus/client_golang to v1.12.2

See merge request !20

Also prevents update goroutines from piling up if they are delayed by
adding a semaphore in startUpdate().

Update golang.org/x/crypto commit hash to ae814b3

See merge request !10