Null previous tx in PhysicalTransactionCursor

such that a potentially large transaction won't need to be resident in heap
while deserializing the next one.

community/kernel/src/main/java/org/neo4j/kernel/impl/transaction/log/PhysicalTransactionCursor.java

@@ -57,6 +57,10 @@ public CommittedTransactionRepresentation get()
     @Override
     public boolean next() throws IOException
     {
+        // Clear the previous deserialized transaction so that it won't have to be kept in heap while deserializing
+        // the next one. Could be problematic if both are really big.
+        current = null;
+
         while ( true )
         {
             if ( !logEntryCursor.next() )