Some application may need to access a pinned map which is not
created by bcc. This example shows how to incorporate such
maps with C++ APIs.

Basically, the application needs to create a TableStorage and
pre-populate it with pinned map before calling BPF constructor.
The bpf program defines this pinned map as an "extern" map.
Signed-off-by: Yonghong Song <yhs@fb.com>