Channels: Speed up clickhouse calculations

[macobo]

The previous has queries proved to be problematic and causing a lot of
CPU overhead. We can improve this by using dictionaries instead.

Benchmarked via this query:

SELECT
  channel,
  count(),
  countIf(acquisition_channel(referrer_source, utm_medium, utm_campaign, utm_source, click_id_param) = channel) AS matches
FROM events_v2
WHERE timestamp > now() - toIntervalHour(48)
GROUP BY channel
ORDER BY count() desc

Before this fix:

query_duration_ms:                                                57960
DiskReadElapsedMs:                                                374.712
RealTimeMs:                                                       2891200.667
UserTimeMs:                                                       2704024.783
SystemTimeMs:                                                     1693.265
OSCPUWaitMs:                                                      90.253
OSCPUVirtualTimeMs:                                               2705709.58

After this fix:

query_duration_ms:                                                4367
DiskReadElapsedMs:                                                454.356
RealTimeMs:                                                       213892.207
UserTimeMs:                                                       199363.485
SystemTimeMs:                                                     1479.364
OSCPUWaitMs:                                                      13.739
OSCPUVirtualTimeMs:                                               200837.37

Note that the new tables are not tracked in our schema as usual as
they're pretty much temporary tables to create the dictionary without
needing to upload files to clickhouse servers.