Upgrade eslint to 7.x

[JasonPierce]

The current version of eslint, 5.x, has known vulnerabilities. An upgrade to 7.x would be beneficial from a security standpoint, as well as a general performance enhancement.

However, node-teradata 1.x supports Node 6.x and 7.x, which cannot run eslint 6+. This issue is therefore targeting milestone 2.0, where we will drop support for node 6.x and 7.x