Constant DEFAULT_SPARSE_TRIE_PRUNE_DEPTH 

pub const DEFAULT_SPARSE_TRIE_PRUNE_DEPTH: usize = 4; // 4usize

Default depth for sparse trie pruning.

Nodes at this depth and below are converted to hash stubs to reduce memory. Depth 4 means we keep roughly 16^4 = 65536 potential branch paths at most.