Refactor req-rep operators to use only ZMQ types

[glopesdev]

This PR refactors the req-rep operators to use only built-in types in ZeroMQ (byte[], string and NetMQMessage). This will allow removing the dependency from the OSC package and keep ZMQ support more agnostic to different serialization protocols.

Other feature highlights:

• Request blocks waiting for the response from the server but can now be asynchronously cancelled.
• Response can now operate in one of two modes:
  • As a source (i.e. with no inputs), it emits a ResponseContext which can be evaluated asynchronously using the new SendResponse operator. This provides the basis for flexibly evaluating responses in full asynchronous fashion while still keeping to the strict sequential nature of the req-rep pattern.
  • As a combinator (i.e. with one message input) it will pair incoming responses with requests using a blocking collection. This allows a cache of responses to be pre-computed and distributed back to clients on-demand.
• Dealer now operates as fully asynchronously in distributing messages and routing back responses. The empty frame delimiter is prefixed by default to emulate the req-rep pattern.
• Router can now also operate in source or combinator mode like the Response operator:
  • As a source (i.e. with no inputs) it works like an asynchronous version of the Response operator where multiple requests may be evaluated simultaneously and responses are correctly paired to clients by using message identities.
  • As a combinator (i.e. with one message input) incoming responses are immediately routed back to peers using each message identity.

Utility operators:

• ToMessage constructs a multiple part message from a sequence of frames or data objects.
• ConvertToFrame constructs a single frame for each data object in the sequence.
• ConvertToString decodes a message frame into a string.
• GetIdentity extracts all identity frames from a multiple part message, up to and including the empty frame delimiter.

[glopesdev]

The last few commits refactor the basic req-rep operators to use a NetMQPoller instead of spinning up a Task object. This is to allow proper asynchronous cancellation. It is not really possible to guarantee that asynchronous disposal of a socket will always break out of the current blocking operation without a poller.