perf scripts python: call-graph-from-sql.py: Separate the database details into a...

perf scripts python: call-graph-from-sql.py: Separate the database details into a class

Separate the database details into a class that can provide different
connections using the same connection information.  That paves the way
for sub-processes that require their own connection.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181001062853.28285-7-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>