Enabled connection pool debugging

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

@@ -78,8 +78,11 @@ public class MessageHelper {
         props.put("mail.imaps.timeout", Integer.toString(NETWORK_TIMEOUT));
         props.put("mail.imaps.writetimeout", Integer.toString(NETWORK_TIMEOUT)); // one thread overhead
 
+        props.put("mail.imaps.connectionpool.debug", "true");
         props.put("mail.imaps.connectionpooltimeout", Integer.toString(3 * 60 * 1000)); // default: 45 sec
 
+        // "mail.imaps.finalizecleanclose"
+
         // https://tools.ietf.org/html/rfc4978
         // https://docs.oracle.com/javase/8/docs/api/java/util/zip/Deflater.html
         if (prefs.getBoolean("compress", true)) {