Fix thread safety in Json.mapper()

[vuriaval]

Closes #4672

Instead of following the suggested approach in the issue, a more elegant method was used, where the initialization is delegated to the JVM code for static initialization.

See https://stackoverflow.com/questions/8297705/how-to-implement-thread-safe-lazy-initialization for a thread about this implementation.
See also https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom for the chosen pattern.

[frantuma]

Thanks!