batch updates to the temporary db when publishing

updates with contents generation were super syscall-heavy. for each path
in a package (so at least 2-4, but ordinarily >4) we'd do a db.Put in
ContentsIndex which results in one syscall.Write. so, for every package in
a published repo we'd have to do *at least* 2 but ordinarily >4 syscalls.
this gets abysmally slow very quickly depending on the available system
specs.

instead, start a batch inside each package and finish it when we are done
with the package. this should keep the memory footprint negligible, but
reduce the write() calls from N to 1.

on one of KDE's servers I have seen update publishing of 7600 packages go
from ~28s to ~9s when using batch putting on an HDD.
on my local system the same set of packages go from ~14s to ~6s on an SSD.
(all inodes in cache in both cases)

deb/publish.go

@@ -599,6 +599,13 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 				}
 			}
 
+			// Start a db batch. If we fill contents data we'll need
+			// 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()
+
 			for _, arch := range p.Architectures {
 				if pkg.MatchesArchitecture(arch) {
 					var bufWriter *bufio.Writer