Increase partial fetch size

app/src/main/java/eu/faircode/email/MailService.java

@@ -84,7 +84,7 @@ public class MailService implements AutoCloseable {
     private final static int CONNECT_TIMEOUT = 20 * 1000; // milliseconds
     private final static int WRITE_TIMEOUT = 60 * 1000; // milliseconds
     private final static int READ_TIMEOUT = 60 * 1000; // milliseconds
-    private final static int FETCH_SIZE = 512 * 1024; // bytes, default 16K
+    private final static int FETCH_SIZE = 1024 * 1024; // bytes, default 16K
     private final static int POOL_TIMEOUT = 45 * 1000; // milliseconds, default 45 sec
 
     private static final int APPEND_BUFFER_SIZE = 4 * 1024 * 1024; // bytes