Streams: Add _flush capability to writable streams

[evansolomon]

Allows writable streams to define a _flush() method that can delay the finish event until it completes.

See #7348

[vkurchatkin]

It looks like there is no need in prefinish after this change

[vkurchatkin]

@markstos is right and this doesn't work for async _flush:

  w._flush = function(cb){
    process.nextTick(function () {
      writtenData.push('flushed');
      cb();
    });
  }

I don't think it's a bug with current Transform implementation, but it's definitely a bug for Writable

[markstos]

Also try an a async implemention of _flush and see if it still completes
before the 'finish' event is fired:

w._flush = function(cb){
 process.nextTick(function () {
  writtenData.push('flushed');
   cb();
 }); 
};

See related discussion here:
#7612

[calvinmetcalf]

took a stab at this as well (haven't opened a pull, but can the branch is at https://github.com/calvinmetcalf/node/tree/write-flush I can open my own pull here but was waiting until it actually worked), as it is it fails 2 of the tests

• simple/test-stream2-writable#' finish does not come before sync _write cb' fails pretty strait forwardly
• simple/test-zlib-params fails due to the following bytes 01 00 00 ff ff a5 ac 3d 34 being appended to the output (wtf?)

[jasnell]

@chrisdickinson @trevnorris ... this never seems to have been completed. Any thoughts on whether this is needed and whether it would need to land in v0.12?

[calvinmetcalf]

there is a more comlete pull to do this at nodejs/node#2314

[jasnell]

Excellent. Closing this then in favor of that. Can reopen this if it becomes necessary