Use bump context for TID bitmaps stored by vacuum

Use bump context for TID bitmaps stored by vacuum

Vacuum does not pfree individual entries, and only frees the entire
storage space when finished with it. This allows using a bump context,
eliminating the chunk header in each leaf allocation. Most leaf
allocations will be 16 to 32 bytes, so that's a significant savings.
TidStoreCreateLocal gets a boolean parameter to indicate that the
created store is insert-only.

This requires a separate tree context for iteration, since we free
the iteration state after iteration completes.

Discussion: https://postgr.es/m/CANWCAZac%3DpBePg3rhX8nXkUuaLoiAJJLtmnCfZsPEAS4EtJ%3Dkg%40mail.gmail.com
Discussion: https://postgr.es/m/CANWCAZZQFfxvzO8yZHFWtQV+Z2gAMv1ku16Vu7KWmb5kZQyd1w@mail.gmail.com