Faster nnimap: nnimap-retrieve-groups-asynchronous

[Simon Josefsson <jas@extundo.com>]

I tried to speed up new mail checking (`g') on IMAP groups.  Startup
uses old technique, but subsequent `g' can be faster.  The speedup
depends on how many groups usually receives new mail each time you
press `g' (the p below).  It might depend on the server as well.  For
me, it is faster against Cyrus IMAPD -- results from other servers
would be interesting.

nnimap-retrieve-groups-asynchronous's value is t

Documentation:
Send asynchronous STATUS commands for each mailbox before checking mail.
If you have mailboxes that rarely receives mail, this speeds up new
mail checking.  It works by first sending STATUS commands for each
mailbox, and then only checking groups which has a modified UIDNEXT
more carefully for new mail.

In summary, the default is O((1-p)*k+p*n) and changing it to nil makes
it O(n).  If p is small, then the default is probably faster.

You can customize this variable.

Defined in `nnimap'.