examples/client_server.jl returns corrupted data with more than one client

[vchuravy]

vchuravy@odin ~/s/UCX (master)> julia --project=. examples/client_server.jl test 2

ERROR: LoadError: TaskFailedException:
AssertionError: String(buffer) == data
Stacktrace:
 [1] start_client(::Int64) at /home/vchuravy/src/UCX/examples/client_server.jl:58
 [2] (::var"#5#7"{Int64})() at ./task.jl:356

...and 1 more exception(s).

[vchuravy]

As seen in #20

This seems to be due to the fact that tagged recv is not per endpoint, but per worker. So we need to mimick what ucx-py does :/