Add synchronisation around map `getOrElse`.

Needed for proper visibility, in case this code
is called on different threads.
(cherry picked from commit 6181d7f57da6c28cc6f6ef0df873a2f7a960ba73)
Merge pull request #610 from dragos/issue/flaky-completion-relevance-test-1001973

Don't add local suffix (one space character) for field completions
Merge pull request #612 from dragos/issue/add-sync-caching-jars

Add synchronisation around map `getOrElse`.
Add an option to show API diffs when building

The incremental compiler can print diffs that explain
why additional files need recompilation. This commit adds
the diffutil.jar and a user-level option.

Fixed #1001952