introduce ability to lock episode

[kierse]

User has requested capability to lock episodes so that the won't be replaced by better quality version after a certain length of time. See http://forums.sabnzbd.org/index.php?topic=2262.msg36655#msg36655

[kierse]

rather than changing how MR works, why doesn't the user just turn off quality management? User preferred high quality episode and placed a schedule delay on medium and low sources. If a medium or low item was downloaded first, he didn't want them replaced by a high quality version that may become available later (essentially locking the episode). In the current implementation when quality management is off, the first episode downloaded will never be replaced/upgraded.