Fix indexing N+1 for issues and merge_requests

What does this MR do and why?

This MR fixes N+1 that we've noticed in production for Elastic::ProcessBookkeepingService. When it calls preload_database_records, we have N+1 for the routes table:

D, [2023-02-02T11:57:51.120110 #2504285] DEBUG -- :   Route Load (31.4ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 35257476 AND "routes"."source_type" = 'Project' LIMIT 1
D, [2023-02-02T11:57:51.155497 #2504285] DEBUG -- :   Route Load (32.4ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 35686473 AND "routes"."source_type" = 'Project' LIMIT 1
D, [2023-02-02T11:57:51.189867 #2504285] DEBUG -- :   Route Load (31.3ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 29415573 AND "routes"."source_type" = 'Project' LIMIT 1
D, [2023-02-02T11:57:51.225319 #2504285] DEBUG -- :   Route Load (31.4ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 28473992 AND "routes"."source_type" = 'Project' LIMIT 1
D, [2023-02-02T11:57:51.263445 #2504285] DEBUG -- :   Route Load (31.3ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 31157236 AND "routes"."source_type" = 'Project' LIMIT 1
D, [2023-02-02T11:57:51.298697 #2504285] DEBUG -- :   Route Load (32.8ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 22349117 AND "routes"."source_type" = 'Project' LIMIT 1
D, [2023-02-02T11:57:51.334635 #2504285] DEBUG -- :   Route Load (31.6ms)  /*application:console,db_config_name:main,console_hostname:console-01-sv-gprd,console_username:dgruzd-rails*/ SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 10612115 AND "routes"."source_type" = 'Project' LIMIT 1

which causes a lot of extra database requests:

source

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