Add ability to delete MapBlocks from map

builtin/game/chatcommands.lua

doc/lua_api.txt

src/database-dummy.cpp

src/database-dummy.h

src/database-leveldb.cpp

src/database-leveldb.h

src/database-redis.cpp

src/database-redis.h

src/database-sqlite3.cpp

src/database-sqlite3.h

src/database.h

src/map.cpp

src/map.h

src/script/lua_api/l_env.cpp

src/script/lua_api/l_env.h