Staging data ingestion and query optimizations

[nitisht]

• Defer the memtable.push flow outside of the write path. For example make the memtable push async / use a separate thread.
• The disk.push should be made aysnc, so server doesn't wait for the event to be written always. Also discussed in Mitigating blocking IO in highly batched payloads #441
• Concat of events (in the memtable) happens at 16384 events in memory. We need to either get rid of concat completely or move it out of ingest / write flow.