Rsocket

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- Removed copying data and metadata into frame during serialization

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- Send **cancel** to responders when requester disconnects and **error** to requesters when requester disconnects
- Fix guide examples to properly cancel responders which use asyncio Task as value source
- Rewrote guide statistics example to use generator instead of task

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- fire_and_forget now only removes the stream id when the future denoting the frame was sent, is done
- API documentation auto generated at rsocket.readthedocs.io
- Request router changes:
- Raise error on empty or None route specified in request router
- Added the following methods to RequestRouter to allow specifying handlers of unknown routes:
- response_unknown
- stream_unknown
- channel_unknown
- fire_and_forget_unknown
- metadata_push_unknown
- Officially support route aliases by using the decorator multiple times on the same method
- Fix value mapping in request router:
-A parameter of any name (not just *payload*) specified on a routed method with a type-hint other than Payload will use the payload_mapper to decode the value
- Any parameter with the type CompositeMetadata will receive the composite metadata

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- Breaking change: Normalized the request_channel method argument names across implementations and added where missing (vanilla, reactivex etc.):
- **local_publisher** renamed to **publisher**
- **sending_done_event** renamed to **sending_done**
- Breaking change: ReactiveX clients will remove empty payload from request_response Observable, resulting in an actually empty Observable
- Bug fix: fixed channel stream being released prematurely when canceled by requester, and responder side still working
- Bug fix: removed cyclic references in RSocketBase which caused old sessions not to be released
- Bug fix: fixed ability for ReactiveX streams and fragmented responses to send payloads concurrently
- CollectorSubscriber : exposed subscription methods directly instead of relying on internal **subscription** variable
- Reactivex server side request_response allowed to return reactivex.empty(). Library code will replace with empty Payload when needed
- Added EmptyStream for use in stream and channel responses
- Tutorial code: release logged out users from global chat data (weak references)

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- Fragmentation fix - empty payload (either in request or response) with fragmentation enabled failed to send
- Breaking change: *on_connection_lost* was renamed to *on_close*. An *on_connection_error* method was added to handle initial connection errors
- Routing request handler:
- Throws an RSocketUnknownRoute exception which results in an error frame on the requester side
- Added error logging for response/stream/channel requests
- Added *create_response* helper method as shorthand for creating a future with a Payload
- Added *utf8_decode* helper. Decodes bytes to utf-8. If data is None, returns None.
- Refactoring client reconnect flow
- Added example code for tutorial on rsocket.io

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- Command line fixes:
- limit_rate argument was effectively ignored. fixed
- Added on_ready callback to RSocketServer. Called when sender/receiver tasks are ready
- Implement ReactiveX (3.0, 4.0) server side handler. Allows to define RequestHandler directly using ReactiveX
- Added sending_done_event argument to request_channel to allow client to wait until sending to server is complete/canceled
- Added find_by_mimetype to CompositeMetadata class. Returns list of relevant items by mimetype
- Breaking Change: Removed RSocketBase class dependency from RequestHandler. It is not longer required as an argument to __init__