Enable use of counter-trees for performance sensitive instrumentation

[hjdivad]

One idea of Kris's was to support counter-trees. These are just like nodes but without time, so they do not even pay the cost of performance.now.

So we'd have nodes with some children that were nodes, and a separate graph of counters with no time information.

[runspired]

Would not be difficult to achieve this, there can even be a marginal amount of time interleaving. We'd just need heimdall.segmentCounters() to push OP_SEGMENT into the events array with the current counter cache.