Prevent concurrent modification of keysWithEvents map

Fixes #86
which means
Fixes sbt/sbt#3687
Fixes sbt/sbt#3695
Fixes sbt/sbt#3775

build.sbt

@@ -36,5 +36,10 @@ val io = (project in file("io"))
     libraryDependencies ++= Seq(scalaCompiler.value % Test, scalaCheck % Test, scalatest % Test),
     sourceManaged in (Compile, generateContrabands) := baseDirectory.value / "src" / "main" / "contraband-scala",
     initialCommands in console += "\nimport sbt.io._, syntax._",
-    mimaPreviousArtifacts := Set(organization.value %% moduleName.value % "1.0.0")
+    mimaPreviousArtifacts := Set(organization.value %% moduleName.value % "1.0.0"),
+    mimaBinaryIssueFilters ++= Seq(
+      // MiMa doesn't understand private inner classes?
+      // method this(sbt.io.PollingWatchService,sbt.io.PollingWatchService#PollingThread,java.nio.file.Watchable,java.util.List)Unit in class sbt.io.PollingWatchService#PollingWatchKey does not have a correspondent in current version
+      exclude[DirectMissingMethodProblem]("sbt.io.PollingWatchService#PollingWatchKey.this"),
+    ),
   )

io/src/main/scala/sbt/io/PollingWatchService.scala

@@ -67,7 +67,7 @@ class PollingWatchService(delay: FiniteDuration) extends WatchService {
 
   override def register(path: JPath, events: WatchEvent.Kind[JPath]*): WatchKey = {
     ensureNotClosed()
-    val key = new PollingWatchKey(thread, path, new java.util.ArrayList[WatchEvent[_]])
+    val key = new PollingWatchKey(path, new java.util.ArrayList[WatchEvent[_]])
     keys += path -> key
     watched += path -> events
     key
@@ -158,14 +158,12 @@ class PollingWatchService(delay: FiniteDuration) extends WatchService {
   }
 
   private class PollingWatchKey(
-      origin: PollingThread,
       override val watchable: Watchable,
       val events: JList[WatchEvent[_]]
   ) extends WatchKey {
     override def cancel(): Unit = ()
     override def isValid(): Boolean = true
-    override def pollEvents(): java.util.List[WatchEvent[_]] = origin.keysWithEvents.synchronized {
-      origin.keysWithEvents -= this
+    override def pollEvents(): java.util.List[WatchEvent[_]] = {
       val evs = new java.util.ArrayList[WatchEvent[_]](events)
       events.clear()
       evs