Don't export the formatted date through the content provider: now exported as the raw Epoch value - date formatting is a UI concern. That makes MessageInfoHolder lighter.
Added semaphore to prevent third party content resolver from exhausting K-9 resources: it won't allow more than 1 cursor to be active at the same time. Concurrent threads (other content resolver invocations) will be suspended until the previous cursor gets released.
Removed dependency from K9 to MessageProvider (and added ability for components to register themselve for being notified when K9 instance is ready)
Moved Intent broadcast code from MessageProvider to K9
Made query() easily extensible using strategy pattern, the discriminant still being the Uri path
Fixed severe design flaw where the message provider would accumulate messages, causing an OutOfMemoryError even when the message provider isn't explicitely invoked.
query() is now synchronized with message retrieval
