tweak buffering in server.js

[jerch]

Part2 fix of #4158.

[jerch]

@Tyriar I'd say - its back on the fast track.

What I changed:

• reduce timeout to 2ms reverted to 5ms (saves ~30% script runtime for output intensive calls)
• introduce a 2^18 chunk limit
• userInput flag to skip buffering after receiving

You might want to check, if the input latency stays low. If not, then there is a catch hidden somewhere 😅