Cache recovery mode, delta encoding, channel medium layer

[FZambia]

Introducing sth like https://pusher.com/blog/introducing-cache-channels/, to help with centrifugal/centrifugo#745 - we now have RecoveryModeCache to move recovery behaviour into the new mode which only recovers latest publication in channel.

Another feature which occasionally araised here is delta compression - possibility to use Fossil algorithm to send delta updates in channels. See https://ably.com/blog/message-delta-compression for a good introduction into general idea here

And one more experimental concept of Channel Medium layer is also introduced here - it's a layer which provides useful per-channel tweaks. Opens a way to send delta updates in at most once scenario, queue and delay broadcast on channel level, optimize position sync in channels with many subscribers.

Would be nice to have separate pull requests for these three features – but it's a bit time-consuming to split at this point :(

[codecov]

Codecov Report

Attention: Patch coverage is 46.66667% with 32 lines in your changes are missing coverage. Please review.

Project coverage is 83.13%. Comparing base (69e4d52) to head (8917541).
Report is 33 commits behind head on master.

❗ Current head 8917541 differs from pull request most recent head 4e575c4. Consider uploading reports for the commit 4e575c4 to get more accurate results

Files	Patch %	Lines
client.go	50.00%	26 Missing and 2 partials ⚠️
options.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
- Coverage   83.70%   83.13%   -0.57%     
==========================================
  Files          37       38       +1     
  Lines        7518     7645     +127     
==========================================
+ Hits         6293     6356      +63     
- Misses        909      965      +56     
- Partials      316      324       +8     

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