Fixed

• modify() operations on larger trees (> 50k entries) that performed multiple
  modification operations could trigger a debug_assert in debug builds, or
  worse, yield incorrect databases in release builds.

  The offending situations occur with edge cases surrounding "absorbing" nodes
  to rebalance trees as entries are deleted. This particular edge case only
  arose when the absorb phase moved entries in both directions and performed
  subsequent operations before the next save to disk occurred.

  This bug should only have been able to be experienced if you were using large
  modify() operations that did many deletions as well as insertions, and even
  then, only in certain circumstances.