Support large object streaming

[yanchengnv]

Fixes # .

Description

This PR implements the following features:

• A general-purpose object streamer.
• A file streamer implemented with object streamer.

The object streaming solution works as follows:

• On the sender side, a generator object is provided that generates objects to be streamed to the targets.
• On the receiver side, a processor factory object is registered on channel/topic. When a stream object is received on the specified channel/topic, the factory object is called to get a new processor, which is used to process objects on the same stream.
• Stream objects are defined as Shareable objects. This makes it possible to pass them through Filters before sending (in future PR).
• Stream objects are sent through aux messages.

For the application to use the object streamer:

• The application must first register processing information on the receiving side by calling engine.register_stream_processing() on certain channel and topic, and provide processor factory and optionally a stream_done callback to be called when the streaming is completed.
• On the sending side, the app calls the engine.stream_shareables() to stream objects. Generator object must be provided, which generates an object to be streamed when called. The generator object is also called to process replies from receivers for each object sent.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.