stats: refactor header files

stats: refactor header files

Performant stat segment scraping involves caching the results of
stat_segment_ls (...) and directly fishing counter data from the
shared-memory segment.

To do that, we need to publish several things previously hidden,
declared in stat_client.c:

o stat_client_main_t typedef
o stat_segment_access_t typedef
o stat_segment_access_start inline function
o stat_segment_access_end inline function

Type: refactor

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I3175e3d1f1fd8ea816336a584565179d1972115c