Remove corrupt package references in db recover

When aptly crashes it is possible to get a corrupt database with a dangling key reference.
This results in an error with 'key not found', eg:

    ERROR: unable to load package Pall example-package 1.2.3 778cf6f877bf6e2d: key not found

This change makes `db recover` fix this situation by removing the dangling references.

system/files/corruptdb.go

@@ -0,0 +1,42 @@
+// This utility corrupts a database by deleting matching package entries.
+// Do not use it outside system tests.
+package main
+
+import (
+	"flag"
+	"log"
+
+	"github.com/aptly-dev/aptly/database/goleveldb"
+)
+
+func main() {
+	var dbPath, prefix string
+
+	flag.StringVar(&dbPath, "db", "", "Path to DB to corrupt")
+	flag.StringVar(&prefix, "prefix", "P", "Path to DB to corrupt")
+	flag.Parse()
+
+	db, err := goleveldb.NewOpenDB(dbPath)
+	if err != nil {
+		log.Fatalf("Error opening DB %q: %s", dbPath, err)
+	}
+	defer db.Close()
+
+	keys := db.KeysByPrefix([]byte(prefix))
+	if len(keys) == 0 {
+		keys2 := db.KeysByPrefix([]byte{})
+		for _, key := range keys2 {
+			log.Printf("Have: %q", key)
+		}
+
+		log.Fatal("No keys to delete")
+	}
+
+	for _, key := range keys {
+		log.Printf("Deleting %q", key)
+
+		if err = db.Delete(key); err != nil {
+			log.Fatalf("Error deleting key: %s", err)
+		}
+	}
+}