fix a comment
Change dhandles to use a singly linked list.  Fix bug in close to
acquire the dhandle lock.  #690
Remove line to set SCHEMA_LOCKED flag instead of taking lock.
Back out 5c6e4e0, we still need to set the flag, that's what the lower
layers check.
Merge pull request #692 from wiredtiger/branch690

dhandle locking during close
Merge branch 'develop' into branch682