Make ChainSelStarvation carry an Enclosed

[jasagredo]

The definition of this tracer, carrying a Time, was basically isomorphic to carrying an Enclosed.

[Niols]

LGTM.

Note that the previous ChainSelStarvation* events carried a time. This was added the first time we saw very short starvations, got extremely, and wanted to be sure to measure their exact length, without potential delays introduced in the tracing system. After discussion with @jasagredo, we decided that it was most likely not necessary at this point; it can always be re-introduced subsequently. The time is of course still written in the starvationVar, because that is used by the BlockFetch logic.

[Niols]

(Impressive that you stumble about this time limited leashing attack bug again, considering how rare it is! It also makes one wonder how we didn't notice it beforehand.)

[nfrisby]

The change seems fine, since it seems unlikely we'll truly need more than the centisecond precision the node's logging infrastructure gives on tracer times in the log.

I have no strong preference either way, but I haven't been the one debugging these error traces. So I'll defer to you and Niols.