Bug 833291 - Part 1/7: correct variable namings. rs=sicking, a=tef+

1. 'STORE_NAME' to 'MESSAGE_STORE_NAME',
2. 'lastKey' to 'lastMessageId',
3. 'objectStore' to either 'messageStore' or 'mostRecentStore'.
4. 'record' to either 'messageRecord' or 'mostRecentRecord'
Bug 833291 - Part 2/7: add thread/participant stores. r=hsinyi, a=tef+
Bug 833291 - Part 3/7: fix saveRecord(). r=hsinyi, a=tef+