Add connection deletion

[congwang09]

Resolves: https://github.com/atlanticwave-sdx/sdx-controller/issues/266

Add a database function to mark object as deleted. Then add a delete function for DELETE connection API.

"DELETE /SDX-Controller/1.0.0/connection/285eea4b-1e86-4d54-bd75-f14b8cb4a63a HTTP/1.1" 200 OK
"DELETE /SDX-Controller/1.0.0/connection/285eea4b-1e86-4d54-bd75-f14b8cb4a63a HTTP/1.1" 404 Not Found
"GET /SDX-Controller/1.0.0/connection/285eea4b-1e86-4d54-bd75-f14b8cb4a63a HTTP/1.1" 404 Not Found

[coveralls]

Pull Request Test Coverage Report for Build 9165499790

Details

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 52.132%

---

Totals
Change from base Build 9163231723:	0.4%
Covered Lines:	738
Relevant Lines:	1446

---

💛 - Coveralls