ENT-2923 - remove db access code from the verification thread pool (#4504)

* ENT-2923 - remove db access code from the verification thread pool

* Remove worker pool for tx verification and disable db access.

* Address code review comments

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

@@ -64,6 +64,22 @@ private val _contextDatabase = InheritableThreadLocal<CordaPersistence>()
 var contextDatabase: CordaPersistence
     get() = _contextDatabase.get() ?: error("Was expecting to find CordaPersistence set on current thread: ${Strand.currentStrand()}")
     set(database) = _contextDatabase.set(database)
+
+private val _prohibitDatabaseAccess = ThreadLocal.withInitial { false }
+
+/**
+ * The logic in the [block] will be prevented from opening a database transaction.
+ */
+fun <T> withoutDatabaseAccess(block: () -> T): T {
+    val oldValue = _prohibitDatabaseAccess.get()
+    _prohibitDatabaseAccess.set(true)
+    try {
+        return block()
+    } finally {
+        _prohibitDatabaseAccess.set(oldValue)
+    }
+}
+
 val contextDatabaseOrNull: CordaPersistence? get() = _contextDatabase.get()
 
 class CordaPersistence(
@@ -115,6 +131,9 @@ class CordaPersistence(
     private val liveTransactions = ConcurrentHashMap<UUID, DatabaseTransaction>()
 
     fun newTransaction(isolation: TransactionIsolationLevel = defaultIsolationLevel): DatabaseTransaction {
+        if(_prohibitDatabaseAccess.get()){
+           throw IllegalAccessException("Database access is not allowed in the current context.")
+        }
         val outerTransaction = contextTransactionOrNull
         return DatabaseTransaction(isolation.jdbcValue, contextTransactionOrNull, this).also {
             contextTransactionOrNull = it