Fix for Mantis #3806.

"PcmBuffer::Clear clears the buffer but does not reset the size. If
Get is called on the same PcmBuffer later on with a size that is the
same as (or less than) it was before the call to Clear, the "else"
branch is taken and the memory of buffer (at the address of nullptr)
is poisoned instead of the necessary allocation being performed. A
memset or memcpy on the returned pointer (nullptr) causes a
segmentation fault."

Now that even Debian stable has picked up version 0.8, we can safely
make this a requirement, and remove a lot of old cruft.

This old version has been obsolete for many years - time to remove it.

Allow reusing Tag instances.

Avoid duplicating the tag.

If no state file is configured, don't chek for state changes.

This destroys data inside PcmDsd and other attributes.  It was once
necessary, but is not anymore.