Backport features from v20

[BGehrels]

• Upgraded dependencies to their must current, compatible version (Spring-boot & co, fahrschein, tokens, tracer)

  • Had to fix two things, because Spring changed their jar dependency graph:
    1. We don't depend on Springs internal MultiValueMap anymore but use Guava instead.
    2. Had to explicitly add spring-boot-starter-web, because it has not been implicitly part of actuator anymore

• back ported the automated Trace creation during snapshots (#116)

• back ported the CODEOWNERS file (#91 + #122)

• back ported #120 make lock-timeout configurable

• thought about backporting the performance tests, too, but since Spring Boot 1 goes End-of-Live 2019-08-01, i don't think we'll have much use for it until then.

• updated the version number in preparation for a new release

[ePaul]

:+1:

[BGehrels]

:+1:

[ePaul]

:+1: