[WF4] Implemented RemoveAllBookmarks method
[WF4] Corrected bookmark resumption handling bugs, simplifying the code.

-Bookmark resumptions are now executed in order ResumeBookmark (..)
called, regardless of where in the tree the calls come from.
-The queue of bookmark resumptions now only starts to be executed when
all activities are blocked. Key to this working is the fact that an
Activity becomes temporarily blocked when one of its bookmarks has a
pending bookmark resumption, even if that bookmark is set to
BookmarkOptions.NonBlocking.
-Added support for activities scheduled from bookmark callbacks
to run before the next pending bookmark resumption is executed.
-Added support for due completion callbacks to run after each bookmark
resumption execution.