Save trace data to an external database

[drmorr0]

Right now the tracer just saves everything in memory, and only dumps out requested data "on-demand" when a user makes an export API call. Maybe the trace should actually be written to a "real" TSDB? Or, maybe it just keeps a certain time window in memory, and then saves a trace of everything to a random S3 bucket after that time window has fallen off?

If we write to an external datastore, what datastore should we use?

Related: #60, #80