Expand compound index on members to include member_role

What does this MR do and why?

Query run by Preloaders::UserMemberRolesInProjectsPreloader is slow. One of the contributing factors is that the database does not pick up the right index. See the investigation for further details: https://gitlab.com/gitlab-org/gitlab/-/issues/386039#note_1234189802.

This MR changes the compound index on members from (user_id, source_id, source_type) to (user_id, source_id, source_type, member_role_id), forcing the database to use the correct index.

Migration output

up

$ rake db:migrate:up:main VERSION=20230111125148                                                                               ─╯
main: == 20230111125148 AddUserIdAndSourceIdAndSourceTypeAndMemberRoleIdIndexToMembers: migrating 
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0792s
main: -- index_exists?(:members, [:user_id, :source_id, :source_type, :member_role_id], {:name=>"idx_members_on_user_and_source_and_source_type_and_member_role", :algorithm=>:concurrently})
main:    -> 0.0068s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:members, [:user_id, :source_id, :source_type, :member_role_id], {:name=>"idx_members_on_user_and_source_and_source_type_and_member_role", :algorithm=>:concurrently})
main:    -> 0.0024s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20230111125148 AddUserIdAndSourceIdAndSourceTypeAndMemberRoleIdIndexToMembers: migrated (0.0984s) 

$ rake db:migrate:up:main VERSION=20230111125150                                                                               ─╯
main: == 20230111125150 DropUserIdAndSourceIdAndSourceTypeIndexFromMembers: migrating 
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0708s
main: -- indexes(:members)
main:    -> 0.0058s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- remove_index(:members, {:algorithm=>:concurrently, :name=>"index_members_on_user_id_source_id_source_type"})
main:    -> 0.0018s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20230111125150 DropUserIdAndSourceIdAndSourceTypeIndexFromMembers: migrated (0.0896s) 

down

$ rake db:migrate:down:main VERSION=20230111125150                                                                             ─╯
main: == 20230111125150 DropUserIdAndSourceIdAndSourceTypeIndexFromMembers: reverting 
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0848s
main: -- index_exists?(:members, [:user_id, :source_id, :source_type], {:name=>"index_members_on_user_id_source_id_source_type", :algorithm=>:concurrently})
main:    -> 0.0069s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- add_index(:members, [:user_id, :source_id, :source_type], {:name=>"index_members_on_user_id_source_id_source_type", :algorithm=>:concurrently})
main:    -> 0.0021s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20230111125150 DropUserIdAndSourceIdAndSourceTypeIndexFromMembers: reverted (0.1029s) 

$ rake db:migrate:down:main VERSION=20230111125148                                                                             ─╯
main: == 20230111125148 AddUserIdAndSourceIdAndSourceTypeAndMemberRoleIdIndexToMembers: reverting 
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0742s
main: -- indexes(:members)
main:    -> 0.0074s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:members, {:algorithm=>:concurrently, :name=>"idx_members_on_user_and_source_and_source_type_and_member_role"})
main:    -> 0.0034s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20230111125148 AddUserIdAndSourceIdAndSourceTypeAndMemberRoleIdIndexToMembers: reverted (0.0956s) 

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