Support postgres backend

[David Welch]

Hey all, looking to add PostgreSQL as a supported backend. I saw a rejected PR from a few years back with the expressed desire for a better schema, but I took a stab at a first step as there wasn't much discussion on what a better output to be + I ran some adhoc performance tests against a slightly converted mysql schema and felt comfortable with the numbers there. 

Opened a PR to start discussing – still need to wire up some unit tests, but I adjusted the maven build to run flyway + generate Jooq classes and fixed all the compilation issues (at least to get the submodule capable of a clean install with no warnings). Building the Storage + Parent modules fail on other modules (kafka / etc) so haven't gotten too far up the stack.

Is there interest in this? Happy to do the legwork, or break it out into a separate community lib (assuming it can be configured externally; been a while since I setup zipkin) – just looking for some feedback and collaboration. 

Thanks :) 

[David Welch]

Apologies – forgot the PR link: https://github.com/openzipkin/zipkin/pull/3460