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Trying to get releases working (and keep snapshots working)

Merge pull request rsocket#154 from ReactiveSocket/requesthandler-int…

…erface

Make RequestHandler an interface

Disable Backup-Request / Retries. (rsocket#102)

* Disable Backup-Request / Retries.

**Problem**
There are bugs related to Backup-Request/Retries subscription.
The remaining part of the code is ok.

**Solution**
Disable the code in the ClientBuilder.

* Address comments

Refactor ReactiveSocketServerHandler to be not shareable. (rsocket#94)

** Problem **
There's a memory leak in `ReactiveSocketServerHandler`, it keep adding entries
in the `duplexConnections` map but never remove them.

** Solution **
Instead of having only one `ReactiveSocketServerHandler`, and manage resources
manually, we let Netty allocate one instance per Channel. Then no resource
management is necessary.

** Modifications **
I refactored all the uage of `ReactiveSocketServerHandler` whithout changing
the logic. I also got rid of the method
```
public void channelReadComplete(ChannelHandlerContext ctx) {
    ctx.flush();
}
```
since we only use `writeAndFlush` in the DuplexConnection (we're sure there's
nothing to flush).

Add missing file Help Unsafe object

Frame instance was referenced longer than required. (rsocket#88)

Problem:

`handleXXX` methods in `Responder` were closing over the passed `requestFrame` and using it later in the lifecycle of request processing.
 `Frame` objects and the underlying buffers are not designed to be retained after the scope of the parent method as these objects are threadlocal and reused. This causes issues when the frame object is referenced later in the request processing (eg: `cleanup()`)

 Solution:

 The only reason frame object was retained was to get the stream Id. This change pre-fetches the `streamId` and uses that from within the processing closure.

 Result:

 No more issue with frame access.

Merge pull request rsocket#84 from ReactiveSocket/error-handling

Added error handling in Responder

Merge pull request rsocket#83 from robertroeser/master

added toString for tracing

adding a callback when a reactive socket is closed (rsocket#82)