Small tweaks to the new caching of OPAM files for pinned packages.

This starts to become useful.
Fix timestamps in debug messages
Speed-up rmdir
Speed-up dowload of dev packages from HTTP repos
Tentative fix for #680

I've tested it against the ocsigen repo and it seems to work. Reviews for Darcs experts are welcome.
Fix display of update messages for local repositories
Turn OpamRepository into a OpamMisc.ABSTRACT type (not very useful as is, but keeping consistency is good)
Fix update of dev packages.

For a random reason, the code doing the update of dev packages was not called anymore.