LogByteSizeMergePolicy over-merges with autoCommit=false and documents with term vectors and/or stored fields

Mark Miller noticed this slowdown (see details in LUCENE-994) in his
app.

This happens because in SegmentInfo.sizeInBytes(), we just run through
all files associated with that segment, summing up their byte sizes.

But in the case of shared doc stores (which happens when
autoCommit=false), this is not quite correct because those files are
shared across multiple segments.

I plan to fix sizeInBytes() to not include the size of the doc stores
when they are shared.