MINOR: clock: make global_now_ms a pointer

MINOR: clock: make global_now_ms a pointer

This is preparation work for shared counters between co-processes. As
co-processes will need to share a common date. global_now_ms will be used
for that as it will point to the shm when sharing is enabled.

Thus in this patch we turn global_now_ms into a pointer (and adjust the
places where it is written to and read from, hopefully atomic operations
through pointer are already used so the change is trivial)

For now global_now_ms points to process-local _global_now_ms which is a
fallback for when sharing through the shm is not enabled.