RFR: JDK-8298277: Replace "session" with "scope" for FFM access

[Per Minborg]

This PR proposes changing variable names and text from "session" to "scope". The proposed changes are only for the FFM API and not other parts like the Vector API and test classes.

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Commit messages:
 - Rename session to scope

Changes: https://git.openjdk.org/jdk/pull/11593/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11593&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298277
  Stats: 189 lines in 26 files changed: 0 ins; 0 del; 189 mod
  Patch: https://git.openjdk.org/jdk/pull/11593.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11593/head:pull/11593

PR: https://git.openjdk.org/jdk/pull/11593