Faster upgrade.

 o Changes strategy from scanning node store to scanning rel store, leading to much less random access.
   Upgrade now requires a single sequential scan through rel store, plus random access into node store,
   one access for each node.
 o Modifies the scanning strategy to use 4K-aligned scan buffer instead of 33b buffer, improves scan speed
   one order of magnitude.
 o Remaining: Memory usage is still very high, need to investigate performance when RAM runs out.
Merge pull request #2099 from jakewins/2.1-fast-upgrade

Faster upgrade.