Implement discovery logic

[tonyespy]

Implement discovery logic: if discovery endpoint hit, call protocoldriver.scan(), process ScanList, and add new devices that match any provisionwatchers.

For more details see the Device Services functional requirements document.

Estimated effort: 5-7d

[tonyespy]

See comment in issue #30.

[tonyespy]

We discussed during today's WG meeting and it was decided to defer SDK-based dynamic device discovery to Fuji. It was felt that someone should review the original design first, and expand the design to also cover black-listing, before we implement anything.

It also should be noted that a DS can implement dynamic device discovery on its own with both C and Go SDKs.

[jpwhitemn]

Agreed per 9/16/2019 WG meeting this is Geneva item with design work in Fuji release. Probably impacting non-backward compatibility.

[chr1shung]

resolved in #448