Note the last added layer/params in caffe.proto to prevent conflicts

Currently, `caffe.proto` has two comment lines that keep track of field numbers for layers and layer parameters. They are intended to create conflicts when additional layers are added simultaneously, ensuring a unique assignment of numbers. Unfortunately this scheme does not work; three-way merging will happily agree to add two layers with the same number so long as they agree on what the next number should be.

This PR proposes a solution by noting the name of the last added layer.