Support for large requests and responses

[kmt]

Linkerd is currently reading whole requests and responses into memory which can can be problematic when those are large. Chunked encoding helps with this but is not an option in some cases.

I'm not an expert in Finagle / Netty but looks like Netty has all the capabilities needed and the lack of support for streaming large requests and responses should be added on Finagle level so it also becomes a feature of linkerd. This particular issue looks relevant: twitter/finagle#538

In any case, regardless of the underlying technicalities, as customers with a requirement for supporting larger requests and responses, we'd be interested to see this feature materialize in linkerd soon.

[koiuo]

It seems, that once the feature is implemented in finagle, linkerd should have support for it almost automatically.

I made a PoC patch for finagle twitter/finagle#740
After that I built latest linkerd against patched finagle.

As a result I was able to route 1000 MiB unchunked request using only 64 MiB heap.