[t:3130] Capture oldest txn starttime at txn begin.

git-svn-id: file:///svn/toku/tokudb@26508 c7de825b-a66e-492c-adef-691d508d4ae1

newbrt/txn.c

@@ -188,6 +188,7 @@ int toku_txn_begin_with_xid (
     if (toku_omt_size(logger->live_txns) == 0) {
         assert(logger->oldest_living_xid == TXNID_NONE_LIVING);
         logger->oldest_living_xid = result->txnid64;
+        logger->oldest_living_starttime = result->starttime;
     }
     assert(logger->oldest_living_xid <= result->txnid64);