If the exchange passed in already has a websocket, don't clobber it

instead of errChannelAlreadySubscribed

Given that some subscriptions have multiple pairs, support that as the
standard.

We deliberately use Equal over Len to avoid spamming the contents of large Slices

Note: This is a naieve implementation because we want to rebase the
kraken websocket rewrite on top of this

* Fixes unauthenticated websocket left as CanUseAuth
* Fixes auth subs happening privately

* Consolidate ProductIDs when all subscriptions are for the same list

Fixes A.Match(B) confusing which is the incoming key and which is B,
and clarifies tests accordingly

Improved example of an alternate key type

Currencies: Fix Pairs.Equal comparing Delimiter

Can't think of a case where we care about case or delimiter when we're
using this new function, so making it Loose since that matches our
current use case.

We were using the "cancel many" facility of the Kraken api.
However since that doesn't actually report errors individually, it seems
saner to just multiplex over it.
We were going to get N+ responses anyway. Might as well send N+ requests

Having SeedAssets in Setup is cruel and unusual because it calls the
API. Most other interactive data seeding happens in Bootstrap.

This made it so that fixing and creating unit tests for Kraken was
painfully slow, particularly on flaky internet.

* Use Websocket subscriptionChannels instead of local slice
* Remove ChannelID - Deprecated in docs
* Simplify ping handlers and hardcodes message
* Add Depth as configurable orderbook channel param
* Simplify auth/non-auth channel updates
* Add configurable Book depth
* Add configurable Candle timeframes

Duplicate of convert_test.go TestTimeFromUnixTimestampDecimal

All parsed times should be in UTC

If we generate one sub for all pairs, but then fan it out in the
responses, we end up with a mis-match between the sub store and
GenerateSubs, and when we do FlushChannels it will try to resub
everything again.

Kraken: Generate N+ subs for pairs

If we generate one sub for all pairs, but then fan it out in the
responses, we end up with a mis-match between the sub store and
GenerateSubs, and when we do FlushChannels it will try to resub
everything again.