After JDK-8321565, when dumping the heap in serial (e.g. via -parallel=1 option for jcmd GC.heap_dump), heap dumper creates two files (e.g. foo.hprof and foo.hprof.p0), then merges the two files at a later time.

This consumes almost double the size of the heap dump, as the disk needs to keep both files alive during merging. This also presents a challenge for systems with limited disk space and require doing serial heap dump to a network socket.

Thus, we should make the heap dumper write to a single file in serial dump mode.