Target current Node.js LTS

[joelpurra]

• Targets current Node.js LTS versions.
  • Remove support for Node.js before v10.
  • Adds support for Node.js v12, v14.
  • Updates the engines field in package.json.
  • ~Updates versions in .travis.yml.~ Uses apt to install (C) library dependencies for Travis tests.
  • Automated testing shows fixes work on Node.js v10, v12, v14, v15 (non-LTS).
  • Manual testing shows fixes work on Node.js v16 (nightly/unreleased).
  • Could remove v10 and add v16 in less than two months from today.
• Fixes C/C++ compilation issues with minimal changes.
  • Relies on Native Abstractions for Node.js (nan) to reduce upgrade efforts for v8 and Node.js core.
  • Should convert to the modern N-API instead, possibly using node-addon-api which should be more future-proof.
• Upgrades dependency versions of C libraries getdns, unbound, openssl as well as npm packages.
• Does not make any efforts towards resolving segfault crashes (#24, #33, possibly more).

Fixes #35.

• https://github.com/nodejs/nan
• https://nodejs.org/dist/latest/docs/api/n-api.html
• https://github.com/nodejs/node-addon-api

[joelpurra]

Note that (nearly) 100% of test runs has at least one segfault and/or uncaught errors which happen outside of the test itself. Travis builds unexpectedly pass anyways, locals tests do not. I will (continue to) leave the fixing to someone with better C/C++ debugging skills.

Released as getdns v3.0.0 on npm.