versioned map list convenience methods

[Bill La Forge]

Add convenience methods to the database for accessing/creating/updating versioned map lists.

[Bill La Forge]

Db.get(id) should return a VersionedMapNode, or null.

[Bill La Forge]

Db.getNil(id) should return a VersionedMapNode, which may be the nil map node.

[Bill La Forge]

Db.clearList(id, key), along with db.clearMap(id), db.remove(id, key, ndx), db.set(id, key, value) and assorted add methods.