Fix imapsync for large mailboxes

Where there is a lot of folders in a mailbox the output may be larger
than TEXT will allow (64KiB). Instead use MEDIUMTEXT (16MiB) which
should be ample.

Additionally check that a sync isn't already running before starting
a new one.

Fixes: #1011

data/web/inc/init_db.inc.php

@@ -364,7 +364,7 @@ function init_db_schema() {
           "automap" => "TINYINT(1) NOT NULL DEFAULT '0'",
           "skipcrossduplicates" => "TINYINT(1) NOT NULL DEFAULT '0'",
           "is_running" => "TINYINT(1) NOT NULL DEFAULT '0'",
-          "returned_text" => "TEXT",
+          "returned_text" => "MEDIUMTEXT",
           "last_run" => "TIMESTAMP NULL DEFAULT NULL",
           "created" => "DATETIME(0) NOT NULL DEFAULT NOW(0)",
           "modified" => "DATETIME ON UPDATE CURRENT_TIMESTAMP",