Finishing up TypeScript transition work

[holgerd77]

TODO List and discussion for finishing up on the TypeScript transition PR at https://github.com/ethereumjs/ethereumjs-devp2p/pull/51.

• [x] Updated TypeScript config on ethereumjs-config and/or in the repo
• [x] Test run on supported Node versions 8, 10, 11 (12 optional)
• [x] Integration test on ethereumjs-client repository, basic benchmarking (2-5 min run comparisons on different protocols)
• [x] Examples test run working confirmation
• [x] Update README (examples, URLs, method signatures)
• [x] Decision on TypeScript definition files (definitions folder), keep or not?

[holgerd77]

Closed by #51 and subsequent work from @PhilippLgh