[dovecot] Thread extension

[Timo Sirainen]

Just figured I'd mention that CVS supports now THREAD extension. I also
did a bit of benchmarking using a folder with 4685 mails (evolution
mailing list):

dovecot+mbox:
 - 0.59s user 0.01s system 98% cpu 0.608 total
 - malloc() memory usage 45072 -> 825685

dovecot+maildir:
 - 0.60s user 0.17s system 98% cpu 0.780 total
 - malloc() memory usage: 45003 -> 825480

Meaning it takes almost 800kbytes of memory and it's quite fast :)
Comparing to other servers then:

uw-imap:
 - 1.11s user 0.27s system 100% cpu 1.376 total
 - malloc() memory usage: 529408 -> ~4348k

courier:
 - 7.27s user 0.28s system 99% cpu 7.567 total
 - malloc() memory usage: 181689 -> ~850k

cyrus:
 - 1.403 total (through TCP connection, I couldn't figure out how to
start it separately)

Whoa, we beat everyone :) Courier uses a bit less memory though, but
that's most likely because we allocate it in larger chunks which is
faster. I ran the tests several times and picked the best values,
although the changes were minimal once they got the messages into OS's
cache.

Also it looks like I've found a couple of bugs in uw-imap and courier's
implementation..