Refactor database code to support standalone batches, transactions.

This is spin-off of changes from #459.

Transactions are not being used yet, but batches are updated to work
with the new API.

`database/` package was refactored to split abstract interfaces and
implementation via goleveldb. This should make it easier to implement
new database types.

deb/publish.go

@@ -612,8 +612,8 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 			// to push each path of the package into the database.
 			// We'll want this batched so as to avoid an excessive
 			// amount of write() calls.
-			tempDB.StartBatch()
-			defer tempDB.FinishBatch()
+			tempBatch := tempDB.CreateBatch()
+			defer tempBatch.Write()
 
 			for _, arch := range p.Architectures {
 				if pkg.MatchesArchitecture(arch) {
@@ -632,7 +632,7 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 								contentIndexesMap[key] = contentIndex
 							}
 
-							contentIndex.Push(qualifiedName, contents)
+							contentIndex.Push(qualifiedName, contents, tempBatch)
 						}
 					}