pull-local: Write detached metadata into the correct repository

We want to write to the dest repo, not src.

Noticed while reviewing this code for some other bug.

https://bugzilla.gnome.org/show_bug.cgi?id=733579
refs: Don't try searching for input strings that can't be objects

I noticed OSTree was a bit slower, did some investigation
and saw we were enumerating all objects for things like

$ ostree rev-parse blah

Since "blah" can never be an object (because of the 'l' and 'h'), just
return no matches.
pull: override max-conns-per-host to 8, for faster downloads

Modern web browsers are all converging on 6 or 8.  Let's match that.
Display bytes/second during pull

Add start time to the AsyncProgress object, use it in the common
progress.
core: Support a per-remote "proxy" configuration option

We don't want to have to force people to set it in the environment.

https://bugzilla.gnome.org/show_bug.cgi?id=733734
pull: Add --old argument, to easily get older commits.