perf scripts python: export-to-postgresql.py: Export switch events

perf scripts python: export-to-postgresql.py: Export switch events

Export switch events to a new table 'context_switches' and create a view
'context_switches_view'. The table and view will show automatically in the
exported-sql-viewer.py script.

If the table ends up empty, then it and the view are dropped.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190710085810.1650-22-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>