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GH-38096: [Java] FlightStream with metadata can cause error when closing (#38110)

### Rationale for this change

The Java FlightStream can raise an error if metadata is transferred and ends up being closed twice.

```
java.lang.IllegalStateException: RefCnt has gone negative
	at org.apache.arrow.util.Preconditions.checkState(Preconditions.java:458)
	at org.apache.arrow.memory.BufferLedger.release(BufferLedger.java:130)
	at org.apache.arrow.memory.BufferLedger.release(BufferLedger.java:104)
	at org.apache.arrow.memory.ArrowBuf.close(ArrowBuf.java:1044)
	at org.apache.arrow.util.AutoCloseables.close(AutoCloseables.java:97)
	at org.apache.arrow.flight.FlightStream.close(FlightStream.java:208)
```

### What changes are included in this PR?

When FlightStream is closed, remove any reference of previous metadata to prevent reference count going negative if closed again. Also added `ExchangeReaderWriter.getResult()` for convenience and clear up ambiguity on error handling.

### Are these changes tested?

Unit tests added for closing with metadata and 

### Are there any user-facing changes?

Added `ExchangeReaderWriter.getResult()`
* Closes: #38096

Authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>