[RUN ALL RSPEC] Move core EE load balancing code to CE

What does this MR do?

This is transparent to the user, we are moving the core code for DB Load balancing from EE to Core.

Not yet moved is code that relies on DB Load Balancing (see #327902 (comment 566635697)). This will be done in subsequent MR

Testing

bin/rspec spec/lib/gitlab/database/load_balancing/* spec/lib/gitlab/database/load_balancing_spec.rb

FOSS_ONLY=1 bin/rspec spec/lib/gitlab/database/load_balancing/* spec/lib/gitlab/database/load_balancing_spec.rb

Related issue: #327902 (closed)

Screenshots (strongly suggested)

EE

Manually tested by running locally with load_balancing configured in config/database.yml:

 11   load_balancing:                                                                                                                                     
 12     hosts:                                                                                                                                            
 13       - /Users/tkuah/code/ee-gdk/postgresql                                                                                                           
 14       - /Users/tkuah/code/ee-gdk/postgresql                                                                                                           
 15       - /Users/tkuah/code/ee-gdk/postgresql 

  (1.5ms)  SELECT pg_current_wal_insert_lsn()::text AS location
 /*application:web,correlation_id:01F6GFRPXGDH4EK5SMA1T9QG7N,line:/lib/gitlab/database.rb:315:in `get_write_location'*/
  ↳ lib/gitlab/database.rb:315:in `get_write_location'
Started GET "/" for 127.0.0.1 at 2021-05-25 12:46:37 +1200
   (7.2ms)  SELECT EXTRACT(EPOCH FROM (now() - pg_last_xact_replay_timestamp()))::float as lag /*application:web,correlation_id:01F6GFSQ9YFE2P1X586G1QB9VN,line:/lib/gitlab/database/load_balancing/host.rb:200:in `query_and_release'*/
  ↳ lib/gitlab/database/load_balancing/host.rb:200:in `query_and_release'
  ApplicationSetting Load (16.2ms)  SELECT "application_settings".* FROM "application_settings" ORDER BY "application_settings"."id" DESC LIMIT 1 /*application:web,correlation_id:01F6GFSQ9YFE2P1X586G1QB9VN,line:/lib/gitlab/database/load_balancing/connection_proxy.rb:94:in `block in read_using_load_balancer'*/
  ↳ lib/gitlab/database/load_balancing/connection_proxy.rb:94:in `block in read_using_load_balancer'

[1] pry(main)> ActiveRecord::Base.connection.class
=> Gitlab::Database::LoadBalancing::ConnectionProxy
[2] pry(main)> ActiveRecord::Base.connection
=> #<Gitlab::Database::LoadBalancing::ConnectionProxy:0x00007fd42be4ded0
 @load_balancer=
  #<Gitlab::Database::LoadBalancing::LoadBalancer:0x00007fd41cded440
   @connection_db_roles={},
   @connection_db_roles_count={},
   @host_list=
    #<Gitlab::Database::LoadBalancing::HostList:0x00007fd42be3e138
     @hosts=
      [#<Gitlab::Database::LoadBalancing::Host:0x00007fd42be45ff0

FOSS

Deleted the ee/ directory, and I don't see load balancing queries anymore :

=> #<ActiveRecord::ConnectionAdapters::PostgreSQLAdapter:0x00007fe8cff4d318
 @advisory_locks_enabled=true,
 @config=
  {:adapter=>"postgresql",
   :encoding=>"unicode",
   :database=>"gitlabhq_development",
   :host=>"/Users/tkuah/code/ee-gdk/postgresql",
   :port=>5432,
   :pool=>11,
   :prepared_statements=>false,
   :variables=>{"statement_timeout"=>"120s"},
   :load_balancing=>{"hosts"=>["/Users/tkuah/code/ee-gdk/postgresql", "/Users/tkuah/code/ee-gdk/postgresql", "/Users/tkuah/code/ee-gdk/postgresql"]}},

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is background work.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done