Avoid using Buffer.concat()

[lpinca]

This patch adds a new function to merge an array of buffers and replaces all occurrences of Buffer.concat() with it.

The reason for this change is that Buffer.concat() checks that every element in the array is an instance of Buffer and this makes it perform worse.

[3rd-Eden]

LGTM

[lpinca]

We should find a way to remove all instanceof checks in the code base. It's unbelievably slow.

[3rd-Eden]

Is it slower than Object.prototype.toString.call(item)?

[3rd-Eden]

And as far as I could see there's only 1, which is in sender.

[lpinca]

Buffer.isBuffer() also uses instanceof internally but yes I think we only have those checks in the sender.

[lpinca]

The fastest way is probably to use thing.constructor === Buffer but it will not work if thing is a subclass.

[lpinca]

Object.prototype.toString() x 1,927,356 ops/sec ±0.99% (82 runs sampled)
Buffer.isBuffer() x 3,755,305 ops/sec ±0.88% (85 runs sampled)
process.binding('util').isUint8Array() x 23,694,865 ops/sec ±1.97% (82 runs sampled)
.constructor x 52,849,107 ops/sec ±2.57% (79 runs sampled)