SFTDenseChunkMap is a small table. It has one entry for each space in the table, and use
side metadata to record the index for each chunk. This works for both 32 bits and 64 bits.
However, its performance is expected to be suboptimal, compared to the sparse chunk map on
32 bits, and the space map on 64 bits. So usually we do not use this implementation. However,
it provides some flexibility so we can set SFT at chunk basis for 64bits for decent performance.
For example, when we use library malloc for mark sweep, we have no control of where the
library malloc may allocate into, so we cannot use the space map. And using a sparse chunk map
will be costly in terms of memory. In this case, the dense chunk map is a good solution.