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.
These are ticks, not timeval, and they're a cause for plenty of files
including time.h just to access now_ms that's only used with ticks
functions. Let's move them over there.
