Make Platform Connection manager configurable - custom executor service.

[dzonekl]

We notice the configuration of the NettyChannelBuilder uses the default executor service.
It is advised to replace it with an executor service, and not use the default, as this will spin more threads when the number of channels grow.

Advise can be seen in i.e. here:
https://www.quora.com/What-are-some-tips-tricks-and-gotchas-when-using-gRPC

We don't have access to the server side code, but wonder how the configuration of grpc is on the server side.