Put the id counter back where a fresh manager starts, so that a namespace emptied and populated again hands out the same generated ids as the first time.
The symbolic name table is deliberately kept: it is the mapping the caller supplied (or
that the first population established) between a name and the id that name must keep, and
a recycled namespace has to honour it again. Nothing can clash with a retired id -
buildNewNodeId skips both the ids in that table and any nodeId still registered.
How node ids are assigned within a namespace.