* Websockets: Move Subscription to its own package
This allows the small type to be imported from both `config` and from
`stream` without an import cycle, so we don't have to repeat ourselves
* Subs: Renamed Currency to Pair
This was being mis-used through much of the code, and since we're
already touching everything, we might as well fix it
* Websockets: Add Subscription configuration
* Binance: Add subscription configuration
* Kucoin: Subscription configuration
* Simplify GenerateDefaultSubs
* Improve TestGenSubs coverage
* Test Candle Sub generation
* Support Candle intervals
* Full responsibility for formatting Channel name on GenerateDefaultSubs
OR consumer of Subscribe
* Simplify generatePayloads as a result
* Fix test coverage of asset types in processMarketSnapshot
* Exchanges: Abstract ParallelChanOp
* Tests: Generic ws mock instances
* Kucoin: Fix intermittent conflict in test currs
Use isolated test instance for `TestGetOpenInterest`.
`TestGetOpenInterest` would occassionally change pairs before
GenerateDefault Subs.
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