Add service to checksum PackageFiles on primary

What does this MR do?

This MR takes care of checksumming every new PackageFile created. This is very first replicable in scope of the new Self-Service Geo framework. Also, we're trying to create the foundation for any new type of replicable here. The feature is under feature flag.

The migration output

UP
== 20200213100530 AddVerificationColumnsToPackages: migrating =================
-- add_column(:packages_package_files, :verification_retry_at, :datetime_with_timezone)
   -> 0.0042s
-- add_column(:packages_package_files, :last_verification_ran_at, :datetime_with_timezone)
   -> 0.0008s
-- add_column(:packages_package_files, :verification_checksum, :string, {:limit=>255})
   -> 0.0020s
-- add_column(:packages_package_files, :verification_failure, :string, {:limit=>255})
   -> 0.0008s
-- add_column(:packages_package_files, :verification_retry_count, :integer)
   -> 0.0007s
== 20200213100530 AddVerificationColumnsToPackages: migrated (0.0087s) ========

DOWN:

== 20200213100530 AddVerificationColumnsToPackages: reverting =================
-- remove_column(:packages_package_files, :verification_retry_count, :integer)
   -> 0.0068s
-- remove_column(:packages_package_files, :verification_failure, :string, {:limit=>255})
   -> 0.0009s
-- remove_column(:packages_package_files, :verification_checksum, :string, {:limit=>255})
   -> 0.0016s
-- remove_column(:packages_package_files, :last_verification_ran_at, :datetime_with_timezone)
   -> 0.0008s
-- remove_column(:packages_package_files, :verification_retry_at, :datetime_with_timezone)
   -> 0.0006s
== 20200213100530 AddVerificationColumnsToPackages: reverted (0.0135s) ========

Screenshots

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

Closes #13838 (closed)