Migrate ZooKeeper Tables from /tables to be organized under the /namespaces hierarchy

Currently, tables in ZooKeeper are stored flatly under `/tables` with no hierarchy. Under each table node is a `/namespace` node that stores the respective namespace that the table is from.

The tables can all be migrated under `/namespaces/` nodes and organized under their respective namespaces. By doing so, the namespace for each table will already be known from the path and no additional lookup to the `//namespace` node is needed. This allows the `//namespace` nodes to be deleted.

Current ZooKeeper Table structure:

- `/tables/namespaceA.tableA, namespaceB.tableB, etc.`

Proposed Migration:

- `/namespaces/namespaceA/tables/tableA`
- `/namespaces/namespaceB/tables/tableB`
- etc.

Relates to #4698 in which the name-to-Id mappings for each Table will be created in near future.