CORDA-3960: Port MultiRPCClient to OS (#6644)

* CORDA-3960: Port MultiRPCClient to OS

* CORDA-3960: Carefully restore serialisation environment in `MultiRpcClientTest` to eliminate side effects on other tests

* CORDA-3960: Move ThreadDumpUtils.kt to `core-utils`

core/src/main/kotlin/net/corda/core/utilities/ThreadDumpUtils.kt

@@ -0,0 +1,91 @@
+package net.corda.core.utilities
+
+import java.lang.management.LockInfo
+import java.lang.management.ManagementFactory
+import java.lang.management.ThreadInfo
+
+fun threadDumpAsString(): String {
+    val mxBean = ManagementFactory.getThreadMXBean()
+    val threadInfos: Array<ThreadInfo?> = mxBean.getThreadInfo(mxBean.allThreadIds, Integer.MAX_VALUE)
+    return "Thread Dump:\n" + threadInfos.filterNotNull().joinToString(separator = "\n") { ti -> ti.asString() }
+}
+
+/**
+ * Inspired by `ThreadInfo.toString`
+ *
+ * Returns a string representation of this thread info.
+ * The format of this string depends on the implementation.
+ * The returned string will typically include
+ * the [thread name][.getThreadName],
+ * the [thread ID][.getThreadId],
+ * its [state][.getThreadState],
+ * and a [stack trace][.getStackTrace] if any.
+ *
+ * @return a string representation of this thread info.
+ */
+fun ThreadInfo.asString(maxFrames : Int = 256): String {
+    val sb = StringBuilder("\"" + threadName + "\"" +
+            " Id=" + threadId + " " +
+            threadState)
+    if (lockName != null) {
+        sb.append(" on $lockName")
+    }
+    if (lockOwnerName != null) {
+        sb.append(" owned by \"" + lockOwnerName +
+                "\" Id=" + lockOwnerId)
+    }
+    if (isSuspended) {
+        sb.append(" (suspended)")
+    }
+    if (isInNative) {
+        sb.append(" (in native)")
+    }
+    sb.append('\n')
+    var i = 0
+    while (i < stackTrace.size && i < maxFrames) {
+        val ste: StackTraceElement = stackTrace.get(i)
+        sb.append("\tat $ste")
+        sb.append('\n')
+        if (i == 0 && lockInfo != null) {
+            when (threadState) {
+                Thread.State.BLOCKED -> {
+                    sb.append("\t-  blocked on $lockInfo")
+                    sb.append('\n')
+                }
+                Thread.State.WAITING -> {
+                    sb.append("\t-  waiting on $lockInfo")
+                    sb.append('\n')
+                }
+                Thread.State.TIMED_WAITING -> {
+                    sb.append("\t-  waiting on $lockInfo")
+                    sb.append('\n')
+                }
+                else -> {
+                }
+            }
+        }
+        for (mi in lockedMonitors) {
+            if (mi.lockedStackDepth == i) {
+                sb.append("\t-  locked $mi")
+                sb.append('\n')
+            }
+        }
+        i++
+    }
+    if (i < stackTrace.size) {
+        sb.append("\t...")
+        sb.append('\n')
+    }
+    val locks: Array<LockInfo> = getLockedSynchronizers()
+    if (locks.isNotEmpty()) {
+        sb.append("""
+	Number of locked synchronizers = ${locks.size}""")
+        sb.append('\n')
+        for (li in locks) {
+            sb.append("\t- $li")
+            sb.append('\n')
+        }
+    }
+    sb.append('\n')
+    return sb.toString()
+}