pb: avoid protobuf warning due to common filename (#1519)

If a Go program imports both badger (v1) and badger/v2, a warning will
be produced at init time:

	WARNING: proto: file "pb.proto" is already registered
	A future release will panic on registration conflicts. See:
	https://developers.google.com/protocol-buffers/docs/reference/go/faq#namespace-conflict

The problem is the "pb.proto" filename; it's registered globally, which
makes it very likely to cause conflicts with other protobuf-generated
packages in a Go binary.

Coincidentally, this is a problem with badger's pb package in the v1 module,
since that too uses the name "pb.proto". Instead, call the file
"badgerpb2.proto", which should be unique enough, and it's also the name
we use for the Protobuf package.

Finally, update gen.sh to work out of the box via just "bash gen.sh"
without needing extra tweaks, thanks to the "paths=source_relative"
option. It forces output files to be produced next to the input files,
which is what we want.