Respect lazy tab load

In Visual Studio 2013 the IDE added a new feature where it will lazily
load the actual data for an item in the running document table.  This
includes the ITextBuffer and ITextView objects.  Unfortunately several
APIs used by VsVim will force load these objects even when not needed.
This change makes the load explicit vs. implicit so the caller can make
the appropriate choice.  At this point all uses respect lazy load.