TxnHandler.getValidWriteIdsForTable optimization for compacted tables

When we do not find open writes for the given validTxnList then we either:

1. do not have any writes on the table - we can return writeIdHwm = 0, and no invalid/aborted writes;
2. we have only compacted writes on the table - we can return writeIdHwm = nextWriteId -1, and no invalid/aborted;
3. we have compacted writes and some invalid writes on the table - we can return the lowest invalid write as a writeIdHwm and set it as invalid.

What the current code does instead is sending writeIdHwm = nextWriteId -1, and sending every write as invalid. This results the same response in case 1-2, but probably a longer list in case 3.

So we have place for some optimizations