Explore: Use SeriesData format for loki/logs (#16793)

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

public/app/features/explore/LogsContainer.tsx

@@ -81,7 +81,6 @@ export class LogsContainer extends PureComponent<LogsContainerProps> {
           data={logsResult}
           dedupedData={dedupedResult}
           exploreId={exploreId}
-          key={logsResult && logsResult.id}
           highlighterExpressions={logsHighlighterExpressions}
           loading={loading}
           onChangeTime={onChangeTime}