Avoid redundant COMMIT calls on page views

Avoid redundant COMMIT calls on page views

These just clear empty transactions and do not do anything
otherwise. No need in doing a few extra RTTs for that.

This also avoids "could not COMMIT" errors when transaction
callbacks do silly things like add more callbacks to outside
DBs. In that case, they either ran their callbacks already or
calling clearSnapshot() will error out.

Change-Id: I4d52c4ef69b02a33adffd8892b98ad1bd5c7d6dc