[MlMt] Moving between messages is slow

[Kai Großjohann]

Hi,

when I use n/p or j/k or cursor up/down to navigate through the list of 
mails, then I notice it takes quite a bit of time, as MailMate tries to 
render each message before moving to the next.  I feel this could be 
optimized by deferring the rendering or skipping it if there is already 
another movement command in the queue.

I use the Gmail keybindings.  I'm on r5018, but I've seen this for a 
long time, it's not something new.

Kai