DiskQueue: use explicitly sized int32_t instead of int in on-disk format.

It was assumed that the payloadSize field in the page header was 32 bits.
This was checked by a static_assert on the size of the PageHeader struct.

This change makes that field size explicit, rather than assuming that
sizeof(int) == sizeof(int32_t).

fdbserver/DiskQueue.actor.cpp

@@ -1066,7 +1066,7 @@ private:
 		};
 		uint64_t seq; // seq is the index of the virtually infinite disk queue file. Its unit is bytes.
 		uint64_t popped;
-		int payloadSize;
+		int32_t payloadSize;
 	};
 	// The on disk format depends on the size of PageHeader.
 	static_assert(sizeof(PageHeader) == 36, "PageHeader must be 36 bytes");
@@ -1703,4 +1703,4 @@ TEST_CASE("performance/fdbserver/DiskQueue") {
 	queue->dispose();
 	wait(queue->onClosed());
 	return Void();
-}
+}