Refs #15: refactor of exposing gcomm stats
1. add gcs_backend_stats in gcs.h to store stats of gcomm and passed to galera.
2. use std::map to store gcomm stats of each layer.
3. append gcs_stat pointer at the end of wsrep_stats_var arrays, and change stats_free in replicator_smm.hpp to non-static member function.
(so allocation and deallocation could be in same compile unit)
4. optimize memory allocation by using a single buffer to store gcomm stats values. and gcomm stats keys are static strings.