Add locked column to Ci::JobArtifact

What does this MR do?

• Adds locked column to Ci::JobArtifact.
• Schedules a partial index on expire_at where locked = 0 to be added asynchronously during a weekend to ci_job_artifacts.

This will allow us to query ci_job_artifacts for unlocked and expired artifacts without needing to join ci_builds and ci_pipelines in order to get this information.

This is part of the work in #327281 (closed) and #322817 (closed) to resolve the issue we have where expired and unlocked job artifacts are not being cleaned up.

Remaining steps after this MR

• Verify scheduled index has been created on gitlab.com.
• Add the index for self-managed using add_concurrent_index.
• Open an MR to propagate the value from Ci::Pipeline to Ci::JobArtifact for new pipelines and artifacts.
• Update DestroyAllExpiredService to use the new column and index.
• Backfill the column.

Database

Migrate up

== 20210826110839 PrepareIndexesForCiJobArtifactsExpireAtUnlocked: migrating ==
== 20210826110839 PrepareIndexesForCiJobArtifactsExpireAtUnlocked: migrated (0.0035s) 

== 20210826120834 AddLockedToCiJobArtifacts: migrating ========================
-- add_column(:ci_job_artifacts, :locked, :smallint, {:default=>2})
   -> 0.0022s
== 20210826120834 AddLockedToCiJobArtifacts: migrated (0.0239s) ===============

Migrate down

== 20210826120834 AddLockedToCiJobArtifacts: reverting ========================
-- remove_column(:ci_job_artifacts, :locked, :smallint, {:default=>2})
   -> 0.0015s
== 20210826120834 AddLockedToCiJobArtifacts: reverted (0.0243s) ===============

== 20210826110839 PrepareIndexesForCiJobArtifactsExpireAtUnlocked: reverting ==
== 20210826110839 PrepareIndexesForCiJobArtifactsExpireAtUnlocked: reverted (0.0018s)

Index to be added asynchronously over a weekend

CREATE INDEX CONCURRENTLY ci_job_artifacts_expire_at_unlocked_idx ON ci_job_artifacts (expire_at) WHERE locked = 0;

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #327281 (closed)