Track the top-level WT_REF before any eviction operation.  This avoids complications if the top-level page moves as part of an in-memory split.  It also avoids a bug during an ordinary split where the wrong parent reference was being updated, leaving the original page locked.
refs #625
Merge pull request #626 from wiredtiger/parent-ref

Track the top-level WT_REF before any eviction operation