mhnsw: refactor FVector* classes

Now there's an FVector class which is a pure vector, an array of floats.
It doesn't necessarily corresponds to a row in the table, and usually
there is only one FVector instance - the one we're searching for.

And there's an FVectorNode class, which is a node in the graph.
It has a ref (identifying a row in the source table), possibly an array
of floats (or not — in which case it will be read lazily from the
source table as needed). There are many FVectorNodes and they're
cached to avoid re-reading them from the disk.