Simplify internal communication / flows, starting with baseoutput

[jiping-s]

Currently all internal communications and code flows are built around primitive go channels and signals, as a result the complexity has exploded especially in baseoutput related to shutdown mechanism:

• There are async sender and receiver to enable pipelining / reduce latency
• There are soft stop, hard stop (abort) to reconnect, and hard stop to shut down.
• Network reading and writing can fail independently and the other side may continue to work (unintended)
• All pending chunks whether in queue or being processed in functions need to be collected for recovery
• The code is difficult to test. Currently the only tests involving real output are in run package and they can't cover any exceptional situations.

Instead of a free form that each piece of code decides how to use channel and signals, there could be some unified framework or library that help making:

• The top-level flow should be visualized, in a declarative form. It needs to cover all exceptional situations because dealing with them is the main cause of complexity.
• The components (e.g. acknowledger or sub unit) should have clear input, output, responsibilities and no implicit side-effects

The internal communication in baseoutput is not performance critical as it works on compressed log chunks