Slow metadata index operations when creating many objects

[tskisner]

The serial volume class uses an SQLite DB as its metadata store. When adding new objects to the volume, this DB is updated. Since we can't predict what the user is doing, each object insertion is a discrete SQL transaction. This takes a fraction of a second, but when first creating a volume and adding thousands of objects, this can be slow.

Note that this does not impact the MPI volume, since in that case metadata operations are stored locally in memory (very fast) and replayed to the main SQLite DB during a metadata sync using a single transaction (also very fast).

One solution is to also use an in-memory metadata store even in the serial case with explicit sync to disk.