perf script: Expose usage of the callchain db export via the python api

perf script: Expose usage of the callchain db export via the python api

This change allows python scripts to be able to utilize the recent
changes to the db export api allowing the export of call_paths derived
from sampled callchains. These call paths are also now associated with
the samples from which they were derived.

- This feature is enabled by setting "perf_db_export_callchains" to true

- When enabled, samples that have callchain information will have the
  callchains exported via call_path_table

- The call_path_id field is added to sample_table to enable association of
  samples with the corresponding callchain stored in the call paths
  table. A call_path_id of 0 will be exported if there is no
  corresponding callchain.

- When "perf_db_export_callchains" and "perf_db_export_calls" are both
  set to True, the call path root data structure will be shared. This
  prevents duplicating of data and call path ids that would result from
  building two separate call path trees in memory.

- The call_return_processor structure definition was relocated to the header
  file to make its contents visible to db-export.c. This enables the
  sharing of call path trees between the two features, as mentioned
  above.

This change is visible to python scripts using the python db export api.

The change is backwards compatible with scripts written against the
previous API, assuming that the scripts model the sample_table function
after the one in export-to-postgresql.py script by allowing for
additional arguments to be added in the future. ie. using *x as the
final argument of the sample_table function.

Signed-off-by: Chris Phlipot <cphlipot0@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1461831551-12213-6-git-send-email-cphlipot0@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>