Opaque data structure for holding a block tree entry.
This is a pointer to an element in the block index currently in memory of the chainstate manager. It is valid for the lifetime of the chainstate manager it was retrieved from. The entry is part of a tree-like structure that is maintained internally. Every entry, besides the genesis, points to a single parent. Multiple entries may share a parent, thus forming a tree. Each entry corresponds to a single block and may be used to retrieve its data and validation status.