Don't honour single read per client isc_nm_read() call in the TLSDNS

Don't honour single read per client isc_nm_read() call in the TLSDNS

This reverts commit f17f5e831be6d07cc7476048444befbf1e06d5b6 that made
following change:

> The TLSDNS transport was not honouring the single read callback for
> TLSDNS client.  It would call the read callbacks repeatedly in case the
> single TLS read would result in multiple DNS messages in the decoded
> buffer.

Turns out that this change broke XoT, so we are reverting the change
until we figure out a proper fix that will keep the design promise and
not break XoT at the same time.