GRPC server doesn't call awaitTermination on shutdown

[tandel-pratik]

The gRPC server offers a https://grpc.github.io/grpc-java/javadoc/io/grpc/Server.html#awaitTermination-- to ensure that the server is properly terminated. Currently only shutdownNow is being invoked:

micronaut-grpc/grpc-server-runtime/src/main/java/io/micronaut/grpc/server/GrpcEmbeddedServer.java

Line 201 in df8a330

server.shutdownNow();

It'd be great to add either an option or change default behavior to call awaitTermination with some configurable timeout.

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[kow3ns]

Here is a PR that addresses the issue we found #314

[graemerocher]

Thanks for the contribution!