Complete redesign of the cache mechanism

Hi,

I've been making some small benchmarks and with big mailboxes I see a noticeable delay when unserializing the cached mailbox array.

So I've modified rcube to instead of storing serialized arrays of the mailbox in the db, it now has a 'messages' where each message (only the header actually) is stored in a row. The perfomance gain is about one and a half second in my development machine. It's still just a test, only uid, subject and the date are stored. Althought it's not optimized in any way (a lot of 'deprecated' code is still being executed).

I'll keep playing with this approach and perhaps supply a patch if I see it really improves performance.

Anyone else interested in this change?

ciao, ivan