This requires splitting up import file phase as separate step in then end,
it should be pretty fast, as it only does file move (hardlink) and
DB update for new checksums.
`PackageDownloadTask` is just a reference to file now. Whole process
was rewritten to follow pattern: download to temp location inside the pool,
verify/update checksums, import into pool as final step.
This removes a lot of edge cases when aptly internal state might be broken
if updating from rogue mirror.
Also this changes whole memory model: package list/files are kept in memory
now during the duration of `mirror update` command and saved to disk
only in the end.
Local package pool now implements more generic package pool API.
The base idea is to never expose full paths to files, so that other
kinds of package pools (e.g. package pool in S3) could be used to implement
the same interface.
Files get into the pool only using `Import` method. `Import` method is
now more smart, it supports moving files into the pool, it can detect if
files reside on the same filesystem and use hardlinking instead of copying.
This will make direct mirror downloads still as fast as they were with previous
version which was performing download directly to package pool.
New package pool doesn't have two things implemented yet:
1. New file placement according to SHA256 or other configured hash
2. Calculate at least SHA256/MD5 for each imported files.
MD5 would be required for S3/Swift publishing
* Drop multi-threaded downloader. It doesn't really belong here -
some places require it, some do not, but it's definitely not the
right place to handle it, as it's being used only when updating
mirrors
* Pass expectedChecksums as pointer, so it's easy to drive `nil` value,
and also downloader can fill back checksums (not implemented right now).
* Break down downloader and tests into more files
* Use pkg/errors instead of fmt
NB: Go `defer` order execution is reverse to the order `defer` statements
are executed.
So before the change, `Drop()` was called before `Close()`, which was no-op.
Change that to explicit order in single func, print errors if they happen.
