Bug 807234 - Temporarily disable the test on a CLOSED TREE. r=orange
Bug 833291 - Part 1/N: correct variable namings.

1. 'STORE_NAME' to 'MESSAGE_STORE_NAME',
2. 'lastKey' to 'lastMessageId',
3. 'objectStore' to either 'messageStore' or 'mostRecentStore'.
Bug 833291 - Part 2/N: add thread/participant stores.