CORDA-2525 - Start pooling classpath scanning and related fixes (#4664) (#4685)

* Start pooling classpath scanning

Quickly patch synchronisation of attachment class loader cache.  Needs a revisit but more complicated due to DJVM.

Annotate away for DJVM

Take ClassGraph utils into their own file so we can exclude for DJVM

Clean up a little

* Daemonize the threads

* Seems to be some concurrency problems with use of ClassGraph.  Using a mutex for now to work around.

(cherry picked from commit 7dc7313fb7)

node-api/src/main/kotlin/net/corda/nodeapi/internal/ContractsScanning.kt

@@ -28,7 +28,7 @@ class ContractsJarFile(private val file: Path) : ContractsJar {
     override val hash: SecureHash by lazy(LazyThreadSafetyMode.NONE, file::hash)
 
     override fun scan(): List<ContractClassName> {
-        val scanResult = ClassGraph().overrideClasspath(singleton(file)).enableAllInfo().scan()
+        val scanResult = ClassGraph().overrideClasspath(singleton(file)).enableAllInfo().pooledScan()
 
         val contractClassNames = scanResult.use {
             coreContractClasses