Add an optional thread local allocation buffer

[wsargent]

Could save some allocation on the backend by storing events internally to a buffer in situations where the timestamp / order is not relevant, like with JFR events.

This could also enable a ringbuffer type solution for dumps without involving SLF4J.

https://www.infoq.com/presentations/monitoring-jdk-jfr/

[wsargent]

The logstash async appender literally uses a ringbuffer, and managing an in-process ringbuffer just complicates the API because it would have to happen pre-statement generation. Just write it to tmpfs.