Initialize new JsonEventWriters when layouting in JsonLayout

[knittl]

The doLayout method used a single JsonEventWriter field in a non-thread-safe way. Consequently, writing logs from multiple threads would corrupt its buffer and send malformed JSON to Loki.

This patch initializes a new JsonEventWriter instance for every event that is layouted, similar to how Logback's PatternLayout is implemented. That way, each thread has its own writer instance (and buffer), guaranteeing that the buffer's content remains valid JSON. Allocated memory of the writer and buffer can be freed immediately after the method has returned.

Alternatives include but are not limited to:

• Synchronizing the method, but this has turned out to be quite slow.
• Using a ThreadLocal to allocate one writer instance per thread. This makes the lifetime of the writers a bit fuzzy and it is not clear when their memory will be freed. With long-lived thread pools, this will keep one writer (with buffer) per thread.
• Using a pool of writers, allowing parallelism of up to N threads (could be made configurable via logback.xml) requiring a constant amount of memory overhead (higher parallelism incurs higher overhead).

Closes #245

[nehaev]

Thanks for your contribution, @knittl! Could you please address the comments above?

[nehaev]

LGTM!