[Java] Remove Batch.EMPTY Shared Objects

[batiati]

Since the introduction of batch.getHeader() by #2481, we must create a new instance per request instead of sharing the same Batch.EMPTY instance.
However, to reduce allocations, the underlying buffer is still a shared empty object.

This PR fixes a problem where concurrent requests with reply.len == 0 were overriding each other's headers.

[sentientwaffle]

LGTM -- thanks for tracking this down!

The original bug can be reproduced with the integration test:

    @Test
    public void testRace() throws Throwable {
        try (final var client2 = new Client(clusterId, new String[] {server.getAddress()})) {
            for (int i = 0; i < 1000; i++) {
                var r1 = client.lookupAccountsAsync(new IdBatch(UInt128.id()));
                var r2 = client2.lookupAccountsAsync(new IdBatch(UInt128.id()));

                var t1 = r1.get().getHeader().getTimestamp();
                var t2 = r2.get().getHeader().getTimestamp();
                if (t1 == t2) {
                    throw new Exception("BUG: " + String.format("timestamp: %d = %d", t1, t2));
                }
            }
        }
    }

...Though that is niche enough that it probably isn't worth adding to the integration test suite.

[sentientwaffle]

If we are going to add this to the integration test suite, let's change the task count to 4 or so... it didn't take that many repetitions to hit the bug, and I don't want to make the test suite slower 😄

[batiati]

Cool, it's about 300ms now!