* plugin/dnstap: various cleanups

A recent issue made me look into this plugin, I suspect various other
cleanups (hopefully deletion of code) can be made as well

Remove identical functions ToClientQuery etc, and just use tap.Message
as the base type in plugin. Keep msg/ for a few helper functions that
may proof useful.

This remove the whole test directory as we will just check the things we
are interested in which gives much better feedback and keeps that code
closer together.

tapwr dir is also not needed, writer_test.go was just duplicating the
tests already done. This moves writer.go to the top directory.

Make the only user of dnstap, the forward plugin, use the newer code
also remove the test, a better test there would be a full e2e test to
see the correct thing happens.

Cleanup the Tapper interface and move it to dnstapio where it belongs,
remove higher level interfaces that are not used. This remove
dnstap.Tapper and dnstap.IORoutines.

Use the standard mechanism for getting access to a plugin and remove
shuffling the plugin into the context.
Signed-off-by: Miek Gieben <miek@miek.nl>

* use opts to get the correct proto
Signed-off-by: Miek Gieben <miek@miek.nl>

* Various fixes
Signed-off-by: Miek Gieben <miek@miek.nl>

* Remove bad addr test, as dnstap is only called from within coredns where these fields have been preparsed
Signed-off-by: Miek Gieben <miek@miek.nl>

* dnstap: remove saving the error

all these fields have been preparsed, no need for dnstap to be pedantic
and check (and save!) this error again.

Simplifies it a bit more.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/forward/dnstap.go
Co-authored-by: Ruslan Drozhdzh <30860269+rdrozhdzh@users.noreply.github.com>

* Code review
Signed-off-by: Miek Gieben <miek@miek.nl>

* add back in preferUDP
Signed-off-by: Miek Gieben <miek@miek.nl>

* nit
Signed-off-by: Miek Gieben <miek@miek.nl>
Co-authored-by: Ruslan Drozhdzh <30860269+rdrozhdzh@users.noreply.github.com>