Optimize worker to worker communication

[vigoo]

For this task we should set up some worker-to-worker communication benchmark based on the worker-executor integration test framework to measure the differences in the various improvements.

All implementations should always provide a protocol version that allows seamless upgrade of the worker-to-worker wire format betwen worker executors, including co-existing multiple formats during deployments.

• A simple step can be to just use a cached pool of gRPC connections instead of always creating a new one
• A possible improvement (to be measured) is to use a bidirectional gRPC stream instead
• Based on the results we can try encoding the wit-value payloads with a different serializer and only use grpc / grpc-streams as envelopes.
• A future step, not to be done as part of this ticket can be to try to reduce the number of conversions in the process, possibly by directly encoding in the stub to the wire format and accessing it from the host function as a linear byte array.