Improve the flow commit API.

Make FinalityFlow do more, and be used more consistently.

Add a new waitForLedgerCommit API that is intended to be used at the end of flows, or at any other point where a flow wants to wait for a transaction to finalise (but the finalisation flow is being done by someone else).

Update the docs a bit.

samples/trader-demo/src/main/kotlin/net/corda/traderdemo/flow/SellerFlow.kt

@@ -55,10 +55,7 @@ class SellerFlow(val otherParty: Party,
                 amount,
                 cpOwnerKey,
                 progressTracker.getChildProgressTracker(TRADING)!!)
-        val tradeTX: SignedTransaction = subFlow(seller, shareParentSessions = true)
-        serviceHub.recordTransactions(listOf(tradeTX))
-
-        return tradeTX
+        return subFlow(seller, shareParentSessions = true)
     }
 
     @Suspendable