plugins/json-tracer: store events async
plugins/json-tracer: store events async

To avoid any possible overhead when a request is traced, we send the
event data on a goroutine now.

This could be channel-based, but the code being 100% local to this
package makes the overhead of channels unappealing compared to simple
mutex locking.

uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server