Usage of default values in protobuf

[joto]

The google protobuf v3 spec doesn't allow you to define default values any more. In v3 defaults are what you would expect: 0 for integers, empty strings for strings, whatever has value 0 in enums.
(For details see https://developers.google.com/protocol-buffers/docs/proto3 and https://github.com/google/protobuf/blob/master/CHANGES.txt#L309).

We are using protobuf v2 currently, but to keep our options open for later switching to protobuf v3 I suggest we get rid of default values that don't correspond to the default values in the protobuf spec in the .proto and move any defaults we need into the vector tile spec.

Specifically this is the version field which has a default of 1 and the extent field which has a default of 4096.