Release 1.2.1

[csvance]

Hey @tanmaykm and @Drvi, in order for gRPCClient.jl 1.0.0 to be released, it needs the recently merged changes from #283

@s-celles also recently developed https://github.com/s-celles/gRPCServer.jl which depends on gRPCClient 1.0.0 for test purposes, but I can't tag the release until I can target the new version of ProtoBuf.jl: JuliaIO/gRPCClient.jl#92