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 https://github.com/parseablehq/parseable/issues/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.