BUG/MINOR: logs/threads: properly split the log area upon startup

BUG/MINOR: logs/threads: properly split the log area upon startup

If logs were emitted before creating the threads, then the dataptr pointer
keeps a copy of the end of the log header. Then after the threads are
created, the headers are reallocated for each thread. However the end
pointer was not reset until the end of the first second, which may result
in logs emitted by multiple threads during the first second to be mangled,
or possibly in some cases to use a memory area that was reused for something
else. The fix simply consists in reinitializing the end pointers immediately
when the threads are created.

This fix must be backported to 1.9 and 1.8.