bug: Incorrect defaults for google.protobuf.Timestamp

[clockworkgr]

Tendermint/Comet BFT treats an undefined/unset Timestamp as a time.Time zero (checking against time.Time.isZero()).

This is 0001-01-01 00:00:00.000.

However, a google.protobuf.Timestamp with { seconds: 0n, nanos: 0 } as defined for undefined/unset here:

https://github.com/confio/cosmjs-types/blob/main/src/google/protobuf/timestamp.ts#L104-L109

results in 1970-01-01 00:00:00.000 (UNIX epoch).

The correct default should be:

{ seconds: -62135596800000n, nanos: 0 }

Admittedly this is golang specific I guess so probably not a good idea to change the "default" encoder but leaving this here in case someone else stumbles across it.

i.e. Always set timestamps