Optimize followed users queries

What does this MR do and why?

This MR optimizes the followed users tabs using the in-operator optimization library.

The change requires an async index because the events table is too big. The change is behind a feature flag: optimized_followed_users_queries

Performance comparison

Note: The measured timings include 160-180ms network latency.

User	Filter	Optimized	Old	% diff
euko	all	750ms	550ms	-36.36%
euko	push	170ms	25230ms	99.33%
euko	merged	180ms	1600ms	88.75%
euko	issue	170ms	8780ms	98.06%
euko	comments	170ms	66200ms	99.74%
euko	team	170ms	1000ms	83.00%
euko	wiki	170ms	270ms	37.04%
euko	designs	180ms	230ms	21.74%
euko	epic	170ms	880ms	80.68%
m_gill	all	370ms	370ms	0.00%
m_gill	push	190ms	135650ms	99.86%
m_gill	merged	190ms	1390ms	86.33%
m_gill	issue	200ms	400ms	50.00%
m_gill	comments	180ms	2150ms	91.63%
m_gill	team	170ms	230ms	26.09%
m_gill	wiki	200ms	190ms	-5.26%
m_gill	designs	200ms	220ms	9.09%
m_gill	epic	190ms	190ms	0.00%
mvanremmerden	all	340ms	540ms	37.04%
mvanremmerden	push	200ms	370ms	45.95%
mvanremmerden	merged	190ms	170ms	-11.76%
mvanremmerden	issue	190ms	190ms	0.00%
mvanremmerden	comments	190ms	290ms	34.48%
mvanremmerden	team	190ms	170ms	-11.76%
mvanremmerden	wiki	180ms	170ms	-5.88%
mvanremmerden	designs	180ms	180ms	0.00%
mvanremmerden	epic	190ms	190ms	0.00%

Database

Example database query:

• old (times out on PRD randomly): https://explain.depesz.com/s/uJmK
• new: https://explain.depesz.com/s/f1Or

Up

== 20220409160628 AddAsyncIndexForEventsFollowedUsers: migrating ==============
-- index_exists?(:events, [:author_id, :target_type, :action, :id], {:name=>"index_events_for_followed_users", :algorithm=>:concurrently})
   -> 0.0046s
-- add_index_options(:events, [:author_id, :target_type, :action, :id], {:name=>"index_events_for_followed_users", :algorithm=>:concurrently})
   -> 0.0001s
== 20220409160628 AddAsyncIndexForEventsFollowedUsers: migrated (0.0187s) =====

Down

== 20220409160628 AddAsyncIndexForEventsFollowedUsers: reverting ==============
== 20220409160628 AddAsyncIndexForEventsFollowedUsers: reverted (0.0076s) =====

Related to #346435 (closed)