Splitstore: support on-disk marksets using badger

[vyzo]

Very useful if you run out of memory during compaction.

[vyzo]

Notes:

• marking is about twice as slow than with the in-memory map, but we have no memory pressure.
• block validation time is unaffected, thanks to the FileIO badger options in the markset.
• removed the markset occurs check from trackTxnRefs so as not to hit the markset; that's a non-critical optimization that does have a small effect in transient memory usage, but much better than stalling i/o access.
• compaction time is about 50% longer than the in-memory mapset, but still under 2hrs
• added some documentation for configuration options in the README.