commandserver: preload repository in master server and reuse its file cache
This greatly speeds up repository operation with lots of obsolete markers:
$ ls -lh .hg/store/obsstore
-rw-r--r-- 1 yuya yuya 21M Dec 2 17:55 .hg/store/obsstore
$ time hg log -G -l10 --pager no
(hg) 1.79s user 0.13s system 99% cpu 1.919 total
(chg uncached) 0.00s user 0.01s system 0% cpu 1.328 total
(chg cached) 0.00s user 0.00s system 3% cpu 0.180 total
As you can see, the implementation of the preloader function is highly
experimental. It works, but I'm yet to be sure how things can be organized.
So I don't want to formalize the API at this point.