Introduced tag versioning.

Introduced tag versioning.

Instead of treating tag as boolean variable (true - present,
false - absent) we now allow it to have many versions. Special
version value 'TAGVER_ZERO' denotes tag absense (corresponds to
'false').

For now, each tag has exactly one version except 'TAGVER_ZERO':
tag's number plus one.

We need tag versions because with only two values it's impossible
to expess more subtle states, e.g. initialized / uninitialized.
Tag versions will also allow to treat fallback tags like any other
tags (in particular, allow them participate in deduplication).
In future, when we add nondeterministic tags, versions will be used
to implement tag copies.