Geo: Add verification state machine

What does this MR do?

• Add a state machine to manage verification state on the primary
• Make VerificationWorker use the state machine to update state
• Query records by state to produce success/failed counts
• This MR prepares us for running workers which constantly query for batches of records that need to be checksummed, and checksums many records simultaneously (see #13839 (closed) for more)

Part of #13839 (closed)

Database migrations

➜  gitlab git:(mk/add-verification-state-machine) ✗ bin/rake db:migrate
== 20201112173532 AddVerificationStateToPackageFiles: migrating ===============
-- add_column(:packages_package_files, :verification_state, :integer, {:default=>0, :limit=>2})
   -> 0.0038s
-- add_column(:packages_package_files, :verification_started_at, :datetime_with_timezone)
   -> 0.0016s
== 20201112173532 AddVerificationStateToPackageFiles: migrated (0.0055s) ======

== 20201112173911 AddIndexOnVerificationStateOnPackageFiles: migrating ========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:packages_package_files, :verification_state, {:name=>"index_packages_package_files_on_verification_state", :algorithm=>:concurrently})
   -> 0.0026s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- add_index(:packages_package_files, :verification_state, {:name=>"index_packages_package_files_on_verification_state", :algorithm=>:concurrently})
   -> 0.0035s
-- execute("RESET ALL")
   -> 0.0002s
== 20201112173911 AddIndexOnVerificationStateOnPackageFiles: migrated (0.0068s) 

➜  gitlab git:(mk/add-verification-state-machine) ✗ bin/rake db:rollback STEP=2
== 20201112173911 AddIndexOnVerificationStateOnPackageFiles: reverting ========
-- transaction_open?()
   -> 0.0000s
-- indexes(:packages_package_files)
   -> 0.0036s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- remove_index(:packages_package_files, {:algorithm=>:concurrently, :name=>"index_packages_package_files_on_verification_state"})
   -> 0.0021s
-- execute("RESET ALL")
   -> 0.0002s
== 20201112173911 AddIndexOnVerificationStateOnPackageFiles: reverted (0.0064s) 

== 20201112173532 AddVerificationStateToPackageFiles: reverting ===============
-- remove_column(:packages_package_files, :verification_started_at, :datetime_with_timezone)
   -> 0.0007s
-- remove_column(:packages_package_files, :verification_state, :integer, {:default=>0, :limit=>2})
   -> 0.0004s
== 20201112173532 AddVerificationStateToPackageFiles: reverted (0.0029s) ======

➜  gitlab git:(mk/add-verification-state-machine) ✗ 

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.