Add "mild" caching for priority calculation.

Enable priority calculation without loading the prev tx from disk using
only a "mild" variant of txprev caching.  This is always enabled,
independently of -txprevcache.
Fix caching of prevPos.
Handle "null" disk tx pos in GetContainingBlock.
Merge pull request #162 from domob1812/simpler-priority

Add "mild" caching for priority calculation.