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