enforce gm GROUP_TABLE abstraction
...by using mnesia:write/3 instead of /1
and generally abstract mnesia reading/writing and make it dirty reads
obvious.
simplify: get rid of tmp var
refactor: tweak and push through gm read_group abstraction
make gm record_new_member_in_group tx side effect free
cosmetic: consistency
refactor: extract gm view change handling
merge stable into default