* Integrate PMMR and its persistent backend with the Chain
* Chain can set tree roots; PMMR backend discard
* Check spent and prune for each input in new block
* Handling of forks by rewinding the state
* More PMMR tests and fixes, mostly around rewind
* Rewrite get_unspent to use the sumtrees, fix remaining compilation issues
* Base MMR storage structures
Implementations of the MMR append-only file structure and its
remove log. The append-only file is backed by a mmap for read
access. The remove log is stored in memory for quick checking
and backed by a simple file to persist it.
* Add PMMR backend buffer, make PMMR Backend mutable
* The Backend trait now has &mut self methods, and an &mut
reference in PMMR. This simplifies the implementation of all
backends by not forcing them to be interior mutable. Slight
drawback is that a backend can't be used directly as long as it's
used by a PMMR instance.
* Introduced a buffer in the PMMR persistent backend to allow
reads before the underlying files are fully flushed. Implemented
with a temporary VecBackend.
* Implement a prune list to use with dense backends
The PruneList is useful when implementing compact backends for a PMMR (for
example a single large byte array or a file). As nodes get pruned and
removed from the backend to free space, the backend will get more compact
but positions of a node within the PMMR will not match positions in the
backend storage anymore. The PruneList accounts for that mismatch and does
the position translation.
* PMMR store compaction
Implement actual pruning of the underlying PMMR storage by
flushing the remove log. This triggers a rewrite of the PMMR nodes
data (hashes and sums), removing pruned nodes. The information of
what has been removed is kept in a prune list and the remove log
is truncated.
* PMMR store pruning tests and fixes
