Allow 0 as a valid external version

Until now all version types have officially required the version to be a positive long number. Despite of this has being documented, ES versions <=1.0 did not enforce it when using the `external` version type. As a result people have succesfully indexed documents with 0 as a version. In 1.1. we introduced validation checks on incoming version values and causing indexing request to fail if the version was set to 0. While this is strictly speaking OK, we effectively have a situation where data already indexed does not match the version invariant.

To be lenient and adhere to spirit of our data backward compatibility policy, we have decided to allow 0 as a valid external version type. This is somewhat complicated as 0 is also the internal value of `MATCH_ANY`, which indicates requests should succeed regardles off the current doc version. To keep things simple, this commit changes the internal value of `MATCH_ANY` to `-3` for all version types.

Since we're doing this in a minor release (and because versions are stored in the transaction log), the default `internal` version type still accepts 0 as a `MATCH_ANY` value. This is not a problem for other version types as `MATCH_ANY` doesn't make sense in that context.

Closes #5662