Zof

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- Update zof for oftr API changes to Port structure, PortMod, and PortStats.

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- Update zof for recent oftr API changes (e.g. ext_hdrs, channel_alert).
- REST API supports POST to /stats/flow/{dpid} to pass JSON parameters to search flows.
- Add serve_forever() method to HTTP api.
- REST API supports a multi-valued query string (using []).
- Add REST API handler for portdesc request to specific port number.

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- oftr changes CHANNEL_UP event to include FeaturesReply with ports.
- oftr renames some events, e.g. REQUEST.FLOW -> REQUEST.FLOW_DESC.
- LLDP support for multiple TLV values with the same subtype.

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- Allow OF messages to be compiled from non-zof tasks.
- Allow zof.ensure_future to be called from outside an async task.
- Update requirements.txt.
- Update http API shim to use aiohttp 3.0 AppRunner API.

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- Add support for 'nxt' and 'ext_hdrs' aliases to PktView.
- CallbackInfo class now allows a base class member to bind to a direct subclass instance.
- Update base alpine image version to v0.48.0-alpine3.7.

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- The contents of the FeaturesReply message is now available from the built-in Datapath object.

- The FeaturesReply message is no longer dispatched separately.