Meta-issue no storage backends: #638

There is some work happening here openzipkin/zipkin#1628

My interest at this time is what features such implementation could provide, i.e.

• what would be write throughput per node with RF=2
• could the backend support indexing of arbitrary tags / log fields, or do they need to be pre-defined
• what is the write amplification or perf impact as a function of # of tags/fields per span
  • in Cassandra backend every tag is an extra write
  • in ES it's extra indexing time on the server
• will the backend support correct server-side joins and LIMIT (broken with Cassandra today)
• how search with multiple tags would be handled
  • in Cassandra it's an AND across different spans from the same service name (weird)
  • in ES it's an AND across tags from the same span only (index document is one span)
• could the backend support latency aggregates out of the box (by service/endpoint)? This one is something I'd expect InfluxDB to be able to do easily, since it's fundamentally a TS db

cc @gianarb @goller