another attempt to fix protobuf-net#442; nerdbank hates me

fix protobuf-net#442 - incr the build version

2.3.17 deploy

2.3.16 release note update

2.3.15 packaging

fix protobuf-net#423 - generated C# for enums in discriminated unions

fix protobuf-net#406 - relative paths for global tool (netcoreapp2.1)

release notes for 2.3.13

Additional fixes for protobuf-net#401; v2.3.12

fix protobuf-net#401 - error in key cache