Consistently use transactions to update database

For any action which is multi-step (requires updating more than 1 DB
key), use transaction to make update atomic.

Also pack big chunks of updates (importing packages for importing and
mirror updates) into single transaction to improve aptly performance and
get some isolation.

Note that still layers up (Collections) provide some level of isolation,
so this is going to shine with the future PRs to remove collection
locks.

Spin-off of #459

aptly/interfaces.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"os"
 
+	"github.com/aptly-dev/aptly/database"
 	"github.com/aptly-dev/aptly/utils"
 )
 
@@ -134,6 +135,9 @@ type Downloader interface {
 	GetLength(ctx context.Context, url string) (int64, error)
 }
 
+// ChecksumStorageProvider creates ChecksumStorage based on DB
+type ChecksumStorageProvider func(db database.ReaderWriter) ChecksumStorage
+
 // ChecksumStorage is stores checksums in some (persistent) storage
 type ChecksumStorage interface {
 	// Get finds checksums in DB by path