[chainstate] memoize block-commit descendancy information for SortitionHandleTx::descended_from

[jcnelson]

The method SortitionHandleTx::descended_from is particularly slow, since it can do lots of MARF reads in the sortition DB. Looking at the log files, it seems to be the biggest contributor to sortition-processing -- even more so than downloading the Bitcoin block! Fortunately, we can easily memoize the PoX anchor block that a block-commit descends from; this should remove the need to query the MARF at all.

[gregorycoppola]

Related: This issue is the line of work suggested in #3014, which suggests looking for ways to cache the MARF in RAM to improve performance.