Improve ci_builds runner_id index

What does this MR do?

See #267374 (closed), specifically this thread

Fixes timeout when viewing runners in admin settings. Adding the id to the index helps with the query on this page. Explain with the following query: https://explain.depesz.com/s/XDxb

SELECT "ci_builds".* 
FROM "ci_builds" 
WHERE "ci_builds"."type" = 'Ci::Build' AND "ci_builds"."runner_id" = 1506021 
ORDER BY id DESC 
LIMIT 30

Before

Time: 40.759 s
  - planning: 0.481 ms
  - execution: 40.759 s
    - I/O read: 40.204 s
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 40814 (~318.90 MiB) from the buffer pool
  - reads: 41120 (~321.30 MiB) from the OS file cache, including disk I/O
  - dirtied: 22114 (~172.80 MiB)
  - writes: 0

After

Time: 148.240 ms
  - planning: 0.470 ms
  - execution: 147.770 ms
    - I/O read: 29.221 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 31794 (~248.40 MiB) from the buffer pool
  - reads: 97 (~776.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

Migration output

== 20201120140210 AddRunnerIdAndIdDescIndexToCiBuilds: migrating ==============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_builds, [:runner_id, :id], {:name=>"index_ci_builds_on_runner_id_and_id_desc", :order=>{:id=>:desc}, :algorithm=>:concurrently})
   -> 0.0106s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- add_index(:ci_builds, [:runner_id, :id], {:name=>"index_ci_builds_on_runner_id_and_id_desc", :order=>{:id=>:desc}, :algorithm=>:concurrently})
   -> 0.0044s
-- execute("RESET ALL")
   -> 0.0001s
-- transaction_open?()
   -> 0.0000s
-- indexes(:ci_builds)
   -> 0.0096s
-- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_on_runner_id"})
   -> 0.0019s
== 20201120140210 AddRunnerIdAndIdDescIndexToCiBuilds: migrated (0.0276s) =====

Rollback output

== 20201120140210 AddRunnerIdAndIdDescIndexToCiBuilds: reverting ==============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_builds, [:runner_id], {:name=>"index_ci_builds_on_runner_id", :algorithm=>:concurrently})
   -> 0.0111s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- add_index(:ci_builds, [:runner_id], {:name=>"index_ci_builds_on_runner_id", :algorithm=>:concurrently})
   -> 0.0151s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- indexes(:ci_builds)
   -> 0.0100s
-- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_on_runner_id_and_id_desc"})
   -> 0.0027s
== 20201120140210 AddRunnerIdAndIdDescIndexToCiBuilds: reverted (0.0401s) =====

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team