…B. (#1486)

(cherry picked from commit dc653b0)

This PR adds a new `db.MaxVersion()` API that returns the
maximum version across all keys in the DB.

(cherry picked from commit 36af85f)

(cherry picked from commit f07dcfc)

Add --num_versions flag to set NumVersionsToKeep option in badger.

(cherry picked from commit c68fbc1)

The untyped const causes conversion to int, for which the value is too large.
Use a typed const instead that is large enough to store value.

Similar to #1541, but for v2.2007 branch.

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.

(cherry picked from commit 3e6a4b7)

Co-authored-by: Daniel Martí <mvdan@mvdan.cc>

(cherry picked from commit 6c1e5bc)