Change WT_CURSOR::insert to not hold a position/resources in the tree on
success.

Add additional wording that WT_CURSOR::reset should be called after any
of the WT_CURSOR search, search-near, update or remove methods, once
the cursor is no longer being used to iterate or retrieve values.

Closes #587.
Have LSM actively reset cursors when not holding a position.
Merge pull request #673 from wiredtiger/cursor-insert-change

Change WT_CURSOR::insert to not hold a position.