This is the first step moving towards Explore supporting logs for 
more datasources than Loki. In the first step we move all the log 
processing from Loki into Explore.
- Make explore convert logs result returned from datasources to SeriesData, 
if needed, and for now convert SeriesData to LogsModel.
- Loki datasource query now returns SeriesData and all
processing have been moved into explore instead.
- Removed key from LogRowModel and use log row indexes as 
the unique key instead.
- Removed id from LogsModel since it looks like it's not in use.
- Introduced a new class FieldCache which is convenient to use when
looking up multiple fields and/or field types and series values.

Closes #16287