RFR (M) 8234510: Remove file seeking requirement for writing a heap dump

[Schmelter, Ralf]

I forgot to post the updated webrev:
http://cr.openjdk.java.net/~rschmelter/webrevs/8234510/webrev.2/

In addition to the changes requested by Thomas, I also renamed the entries in the heap dump segment from entries to sub-records, since that is what they are called in the comment describing the format.

Best regards,
Ralf