RFR: 7266: Context Switch event handling breaks if JMC has old and new types parsed simultaneously

[Henrik Dafgård]

This PR fixes an issue where the context switch rate attribute isn't final and can thus break if different types of profiles are parsed in sequence while still being read.

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

Commit messages:
 - Use constant attribute for switch rate

Changes: https://git.openjdk.java.net/jmc/pull/262/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=262&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7266
  Stats: 29 lines in 5 files changed: 13 ins; 14 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/262.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/262/head:pull/262

PR: https://git.openjdk.java.net/jmc/pull/262