Add caching support in rcube_imap::get_uid/get_id methods

[alec]

These methods are using only FETCH command or internal map. When we'd use DB cache also, e.g. in 'show' action 4 FETCHes will be raplaced with 4 SELECTs.

[neek]

Proposal patch attached. The db cache consists in a hash uid => id per mailbox. Let me know if this is what you were looking for.

[alec]

I was thinking about to use message table, which contains idx and uid. Using it will be much better.

[neek]

Patch using message_cache_index

[neek]

Hi Alec,

Take a look at this new patch when you have some minutes. It now uses get_message_cache_index that makes any uid/idx lookup to hit icache array.

Thanks,
Adolfo.

[alec]

There maybe a situation when internal cache is not set. In this case fetching the whole index array is not good. I've improved this a little bit in [29983c16].