Refs #15: measure message replication latency in evs input_map
Refs #15: add interface to expose gcomm stats to gcs
1. since evs_proto already measure replication message in validate_reg_msg, remove previous commit
2. put gcomm stats into NULL-terminated array. for simplicity, all stats are string.
3. rewrite galera stats_get returned buffer, to support variable stats vars

Conflicts:
	galera/src/replicator_smm_stats.cpp
	gcomm/src/gcomm/protolay.hpp
	gcs/src/gcs.cpp
	gcs/src/gcs.hpp
	gcs/src/gcs_dummy.cpp
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.

Conflicts:
	galera/src/galera_gcs.hpp
	galera/src/replicator_smm.hpp
	gcomm/src/gcomm/protolay.hpp
	gcs/src/gcs.cpp
	gcs/src/gcs.hpp
	gcs/src/gcs_dummy.cpp
Refs #15: fix typo
Refs #15: move gcomm::histogram code to gu

Conflicts:
	galerautils/src/SConscript
	galerautils/src/gu_histogram.cpp
	galerautils/tests/SConscript
	galerautils/tests/gu_tests++.hpp
Refs #15: add gu::stats to compute avg and std dev online.
1. gu::histogram contains gu::stats so it will compute avg and std dev automatically.
2. change output format of message replication latency by using output of gu::stats.
Refs #15: add min/max to gu::stats