RFR: 8297505: Declare fields in some sun.security.pkcs11 classes as final

[Per Minborg]

This PR proposes declaring some fields as `final` to potentially unlock performance optimisations and improve thread visibility (e.g. for GC threads). The PR also fixes a "synchronising on a non-final field" anti-pattern.

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Commit messages:
 - Declare fields and classes final

Changes: https://git.openjdk.org/jdk/pull/11346/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11346&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297505
  Stats: 25 lines in 10 files changed: 0 ins; 0 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/11346.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11346/head:pull/11346

PR: https://git.openjdk.org/jdk/pull/11346