refactor charge sockets data processing into dedicated class

While here, add method to generate the OSM keys corresponding
to a socket list.

Signed-off-by: Séverin Lemaignan <severin@guakamole.org>

libs/indexer/CMakeLists.txt

@@ -158,6 +158,7 @@ target_link_libraries(${PROJECT_NAME}
   geometry
   protobuf
   coding
+  feature_helpers
 )
 
 omim_add_test_subdirectory(indexer_tests)