Replace Gson with Jackson in Onionoo

This ticket should take the proof-of-concept branch from the parent ticket and replace gson thoroughly. Steps:

• remove gson from dependencies
• adapt existing tests, which check part of the escape/unescape issues
• look at the possible pitfalls b/c of different escape strategies (cf. parent ticket for more)
• all touched classes should replace string concatenation in log statements in a separate commit (a step more towards to Metrics coding standards)
• clean-up proof-of-concept branch to not print stack-traces and the like