Revisit output buffer flushing

[WJCIV]

Output is currently going into buffers. We need to spend more time figuring out when to flush those buffers. Doing so after every write is inefficient and undermines the point of using a buffer in the first place, but not flushing often enough will add latency.

[pwatchara]

It is possible to have a case where an upstream operator infrequently sends tuples to its downstream(s) and this may result in high latency due to tuple accumulation in the output buffer(s).

Is there an alternative way to address this, given that currently writing WritableByteChannel is done only after the buffer is detected full (OutputBuffer.bufferIsFull()) ?