RFR: JDK-8261297: NMT: Final report should use scale 1

[Thomas Stuefe]

If PrintNMTStatistics is specified, we write a NMT report to stdout before exiting the VM. This is useful for leak analysis.

However, the report uses the standard "K" scale, which is shouldn't, since this omits small leaks < 1K. Hence it should use scale=1. This would provide us with a precise exit report. Note that for this to work JDK-8261238 has to be fixed too, which is a separate issue.

This patch:
- changes the scale of the final report to 1
- does some minor cleanup work in the MemReporter class hierarchy (const-ifying scale and the output stream, and establishing 0 as "default scale" as to avoid having the default scale hard coded in four places.

-------------

Commit messages:
 - Start

Changes: https://git.openjdk.java.net/jdk/pull/2450/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2450&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261297
  Stats: 36 lines in 3 files changed: 13 ins; 5 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2450.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2450/head:pull/2450

PR: https://git.openjdk.java.net/jdk/pull/2450