Make use of the new size hint functions in btcwire.

This commit changes a couple of sections which deal with large lists of
inventory vectors to use the new size hint functions recently added to
btcwire.  This allows a bit more efficiency since the size of the list is
known up front and we can therefore avoid dynamically growing the backing
array several times.  This also helps avoid a Go bug that leaks memory on
appends and GC churn.