Split do_blocking_move() to XY and Z sections

do_blocking_move() function might have exceeded the Z homing speed if the distance between two probes is sufficiently small. For some printer constructions (i.e., the ones with tiny beds), this will cause the Z axis to try to move faster than it can, leading to bad things.

This commit splits the Z and XY movement, and uses the Z homing speed for Z movement, while XY_TRAVEL_SPEED is used after the probe is lifted from the bed.