[6.0.0] Header.checkPow method

[kushti]

In this PR, a new method added to check Ergo header's proof-of-work

{
     val h = CONTEXT.headers(0)
      h.checkPow
 }

[kushti]

JS test was failing due to #1023 , fixed now

[aslesarenko]

This will be the first code fragment versioned by tree version (rather than activatedVersion). Maybe use activatedVersion instead? dApps usually don't care about versions, and hence requiring a minimal tree version may require dApp upgrades.

[kushti]

Possible, however, there are some questions to resolve then :

• NewFeature implementation by you was expecting this:

    override def isSupportedIn(vc: VersionContext): Boolean =
      vc.activatedVersion >= sinceVersion && vc.ergoTreeVersion >= sinceVersion

why ?

• what does tree version mean at all ? How tree of version v+1 is different from tree version v?

[kushti]

Rollback versioning by tree and changed NewFeature definition. The question about versioning semantics still remains.

[aslesarenko]

This "vc.activatedVersion >= sinceVersion && vc.ergoTreeVersion >= sinceVersion" was an intermediate version, which was fixed in the in my original more recent PRs.
The tree version is more about bytecode format, see for example switch from v0, to v1, there size has become required. However, there is also a relation between tree version and block version, s.t. treeVersion <= activatedScriptVersion.

[kushti]

@aslesarenko comments addressed, please make another pass

[aslesarenko]

LGTM, also see comment about tree version.