Add predownload plugin

[andrii-suse]

Purpose:

• get faster downloads utilizing concurrent downloads with sypplite utility
• let others experiment with custom implementation of download (e.g. to deal with their specific limitations)

Implementation algorithm:

• check if any plugin present;
• create list of Transaction Steps which require download;
• for each repository involved in future downloads: send to plugin a string where first word is baseurl and the tail is list of files required for download from that repo. (command PREDOWNLOAD_FROM_REPO);
• if any of plugins is still present: pick the first one and ask him to do actual download (command PREDOWNLOAD_START);
• check plugin for progress every second (command PLUGIN_PROGRESS);
• All packages are expected to be in the cache with a suffix ".unverified" now;
• The fetcher code now is aware of ".unverified" suffix and tries to look up and verify such files;

Implementation details:

• a new method progress() was added to PluginScript, which defines how plugins will report progress;
• the current implementation of the plugin sypplite.sh will create request files for sypplite utility in /var/cache/zypp/packages/ .

Testing with prebuilt libzypp (Tumbleweed only so far):

zypper ar -f http://download.opensuse.org/repositories/home:andriinikitin:sypper/openSUSE_Tumbleweed home:andriinikitin:sypper
zypper --gpg-auto-import-keys  ref
zypper -n in --from home:andriinikitin:sypper sypplite libzypp
cp /usr/share/sypper/predownload/sypplite.sh /usr/lib/zypp/plugins/predownload/

Current experimented results for performance gain on 200 - 3000 packages is 40% - 200%

[lkocman]

40% is super impressive!

[lkocman]

I'm not sure what's the teams capacity, but using a plugin and over the time merge functionality into the base libs could be a good short term solution. This woud allow us to aim at 16.0 Beta and similar.