CORDA-3471: Create CordaTransactionSupport and make it accessible through AppServiceHub (#5768)

* CORDA-3471: Create `CordaTransactionSupport` and use wherever possible instead of `CordaPersistence`

* CORDA-3471: Address comments by @mnesbit
- Relocate `CordaTransactionSupport` to `core`
- Create a lighter version of transaction - `VaultTransaction` that gives access to `session` object only.

* CORDA-3471: More changes after discussion with  @mnesbit
- Rename `VaultTransaction` into `SessionScope`.

* CORDA-3471: Revert changes to most of the files after conversation with @mnesbit and @rick-r3

* CORDA-3471: Introduce `CordaTransactionSupportImpl` and make it accessible via `AppServiceHub`.

* CORDA-3471: Minor change (comment).

* CORDA-3471: Address input from @mnesbit

* CORDA-3471: Address input from @rick-r3

* CORDA-3471: Make Detekt happier

* CORDA-3471: Add a new test that proves transactions can be started from client threads

As requested by @mnesbit

* CORDA-3471: Change log and documentation update.

As requested by @mnesbit

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

@@ -0,0 +1,21 @@
+package net.corda.nodeapi.internal.persistence
+
+import net.corda.core.node.services.vault.CordaTransactionSupport
+import net.corda.core.node.services.vault.SessionScope
+
+/**
+ * Helper class that wraps [CordaPersistence] and limits operations on it down to methods exposed by [CordaTransactionSupport].
+ */
+class CordaTransactionSupportImpl(private val persistence: CordaPersistence) : CordaTransactionSupport {
+    override fun <T> transaction(statement: SessionScope.() -> T): T {
+        // An alternative approach could be to make `DatabaseTransaction` extend from `SessionScope`, but this will introduce a hierarchical
+        // dependency which might be unwanted in some cases.
+        fun DatabaseTransaction.innerFunc(): T {
+            return statement.invoke(
+                object : SessionScope {
+                    override val session = this@innerFunc.session
+                })
+        }
+        return persistence.transaction(0, DatabaseTransaction::innerFunc)
+    }
+}