Migrate to Protobuf 3 to Increase Language Compatibility

Some languages, most notably C#, do not support proto2. The proto files for comdb2 use several specific features which have been deprecated in proto3, like optional values, defaults, required values, non-base-0 enums.

So, this is a feature request to migrate to proto3. The protobuf interface is an easy way to talk to the database and migrating will path the way for more adoption, e.g. a proper database driver for C#.

The issue identifies the comdb2 proto files and the proto2 features that must be addressed, including optional, default, required values and non-base-0 enums. Start by reviewing those definitions and the current protobuf interface; done means the interface is migrated to proto3 while preserving a usable way to talk to the database and improving C# compatibility.

Written by the indexing model from the issue text.