Don't store state from ClientBootstrap.channelInitializer (#439)
Don't store state from ClientBootstrap.channelInitializer (#439)

- channelInitializer can be called multiple times (happy eyeballs)
- the multiplexer was captured but it was never guaranteed to be from
  the "winning" connection
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift