Do not block main thread at startup with long running threads

• Pulling large repos block main thread
• Move pulling to background so that main thread returns fast and service becomes healthy
• Hold state if pulling is finished (see SonarQube that shows a waiting page)

14:12:43.508 [main] WARN  o.g.s.c.c.GitRepositoriesConfiguration - Pulling of 4 git repositories may take a while
14:12:43.523 [pool-1-thread-1] DEBUG org.gesis.commons.git.JGitRepository - git pull master in /stardat-converter/working-directory/dbk-ddi32 from ssh://git@svko-docker2.gesis.intra:3022/gesis/dbk-ddi32.git
14:12:43.541 [pool-1-thread-3] DEBUG org.gesis.commons.git.JGitRepository - git pull master in /stardat-converter/working-directory/dsdm-ddi32 from ssh://git@svko-docker2.gesis.intra:3022/gesis/dsdm-ddi32.git
14:12:43.544 [pool-1-thread-2] DEBUG org.gesis.commons.git.JGitRepository - git pull master in /stardat-converter/working-directory/dbk-ddi32-extended from ssh://git@svko-docker2.gesis.intra:3022/gesis/dbk-ddi32-extended.git
14:12:43.546 [pool-1-thread-4] DEBUG org.gesis.commons.git.JGitRepository - git pull master in /stardat-converter/working-directory/das-ddi32 from ssh://git@svko-docker2.gesis.intra:3022/gesis/das-ddi32.git
14:21:57.116 [main] INFO  o.a.coyote.http11.Http11NioProtocol - Starting ProtocolHandler ["http-nio-8080"]