shill: Added move operators to ByteString and IPAddress.

This change address move assignment operator and move constructor to
ByteString and to IPAddress.

ByteStrings can become very large and copying them can become expensive
operations.  IPAddress can similarly benefit.  By providing move
operators to them, they can be efficiently passed.

TEST=unittest
BUG=chromium:863260

Change-Id: I8faf0e06d5cadabd6407bb544ff25e73ee0a9321
Reviewed-on: https://chromium-review.googlesource.com/1170562
Commit-Ready: Alex Dale <sigquit@google.com>
Tested-by: Alex Dale <sigquit@google.com>
Reviewed-by: Stephen Barber <smbarber@chromium.org>