make id's global in buffer schema

[mk30]

right now, the same id can exist under a node, way, and relation. buffer schema should be modified to make id's global.

[mk30]

can: multiply id by 3 and add 0 for node, 1 for way, 2 for relation.

to get out the original id: divide enhanced id by 3 and take the floor.
to get the type: enhanced id modulo 3.