* Cleanup MMRs positional indexes. Get rid of the kernel position index.
Introduced a new block marker index that keeps, for each block, the
respective positions in the output and kernel MMRs. This is now sufficient
for rewind.
* Block marker compaction
* Implementation of compaction for the chain. Single entry point on the chain triggers compaction of all MMRs as well as the cleanup of the positional index and full blocks.
* API endpoint, additional tests and more fixes for compaction
* Also prune PMMR metadata, minor bug fix
* PMMR store tests fix
