Handle pageination in Riak search

[shamrickus]

What does this PR (Pull Request) do?

• This PR fixes TO riak client is unable to get more than 1000 records #5902

I have written the tests for this change, but given that it takes ~8 minutes I've decided to omit it.

Which Traffic Control components are affected by this PR?

• Traffic Ops
• Traffic Vault

What is the best way to verify this PR?

Create >1000 DS with sslkeys on a single cdn, call cdns/name/{name}/sslkeys and verify it returns more than 1000 records.
Create >1000 SSLKeys on a single CDN (doesn't matter if it's one or many DS'). Then make a change (such as moving a DS to another CDN) that modifies more than 1000 ssl certs the cdn. Finally, perform a snapshot. Only 1000 of the SSLKey records will have been removed/updated.
The order in which these are returned is transient as far as I can tell, so which 1000 are changed differs each run.

If this is a bug fix, what versions of Traffic Control are affected?

• every version

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[mattjackson220]

Looks good to me! code looks good, tested locally in CiaB and works as expected.

[ocket8888]

TO Go client/API integration tests will pass if you rebase