Switch to ConcurrentHashMap instead of manually locking.

Experienced deadlocks when using the plugin,
Original purpose of synchronized blocks was to prevent getting
  the same resource from the server twice if it wasn't necessary.
  However, duplicating get work is preferable to locking up IntelliJ.

In other words, concurrency is hard.
Update readme.
Update version to 1.0.6 in preparation for release.
Merge branch 'master' into 11.x
Update prebuilt 11.x version.