[Orgmode] Refile target caching

[Carsten Dominik]

Hi Sebastian, hi Samuel,

I remember that both of you have in the past reported that refiling
has a long startup time because of target collection.

I have now built a cache for refile targets and would like you to try  
it out.

(setq org-refile-use-cache t)

This will speed up refile target collection for the second and further  
instance.
If you are moving or adding entries that are targets themselves, that  
chace needs to be cleared with prefix arg 0 (zero), i.e. `C-0 C-c C-w'  
or, if you prefer, with a triple C-u prefix.

Samuel, note that this only speeds up target collection - it does  
nothing to the overhead added by ido - so we will have to see how much  
this helps for your use-case.


- Carsten