MINOR: debug: bump the dump buffer to 8kB

MINOR: debug: bump the dump buffer to 8kB

Now with the improved backtraces, the lock history and details in the
mux layers, some dumps appear truncated or with some chars alone at
the beginning of the line. The issue is in fact caused by the limited
dump buffer size (2kB for stderr, 4kB for warning), that cannot hold
a complete trace anymore.

Let's jump bump them to 8kB, this will be plenty for a long time.