No limit on number of active spans kept in memory by zpages SpanStore

[semistrict]

We should not keep an unbounded number of spans in the active set in spanStore. If a user starts a span without ending it, this will cause a memory leak.

Yes, this is working around potential bugs in user code. But we have actually seen this in the wild (in the Datastore custom spans) and the only reason this didn't cause crashes is that the zpages are not automatically imported. Anyone importing zpages would see a memory leak.