Fix bug in prefix B+tree
Fix bug in prefix B+tree

- When index node is split, splitter for its parent node can be
incorrect if we use the longest common prefix as a splitter.
- In index node, just use the 2nd node's smallest key as splitter for
parent node

hisundar/forestdb,joliss/forestdb,yyangpan/forestdb,couchbase/forestdb,seem-sky/forestdb,sushmaad/forestdb,yyangpan/forestdb,wurikiji/forestdb,wurikiji/forestdb,joliss/forestdb,zhaog/forestdb,uvenum/forestdb,uvenum/forestdb,seem-sky/forestdb,hisundar/forestdb,couchbase/forestdb,sushmaad/forestdb,jinxuan/forestdb,nvoron23/forestdb,nvoron23/forestdb,zhaog/forestdb,jinxuan/forestdb