Set connection pool size to 1

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

@@ -130,7 +130,7 @@ public class EmailService implements AutoCloseable {
 
     private final static int SEARCH_TIMEOUT = 90 * 1000; // milliseconds
     private final static int FETCH_SIZE = 1024 * 1024; // bytes, default 16K
-    private final static int POOL_SIZE = 2; // connections
+    private final static int POOL_SIZE = 1; // connections
     private final static int POOL_TIMEOUT = 90 * 1000; // milliseconds, default 45 sec
 
     private final static int TCP_KEEP_ALIVE_INTERVAL = 9 * 60; // seconds