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.

database/goleveldb/database.go

@@ -0,0 +1,58 @@
+package goleveldb
+
+import (
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/filter"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+	leveldbstorage "github.com/syndtr/goleveldb/leveldb/storage"
+
+	"github.com/aptly-dev/aptly/database"
+)
+
+func internalOpen(path string, throttleCompaction bool) (*leveldb.DB, error) {
+	o := &opt.Options{
+		Filter:                 filter.NewBloomFilter(10),
+		OpenFilesCacheCapacity: 256,
+	}
+
+	if throttleCompaction {
+		o.CompactionL0Trigger = 32
+		o.WriteL0PauseTrigger = 96
+		o.WriteL0SlowdownTrigger = 64
+	}
+
+	return leveldb.OpenFile(path, o)
+}
+
+// NewDB creates new instance of DB, but doesn't open it (yet)
+func NewDB(path string) (database.Storage, error) {
+	return &storage{path: path}, nil
+}
+
+// NewOpenDB creates new instance of DB and opens it
+func NewOpenDB(path string) (database.Storage, error) {
+	db, err := NewDB(path)
+	if err != nil {
+		return nil, err
+	}
+
+	return db, db.Open()
+}
+
+// RecoverDB recovers LevelDB database from corruption
+func RecoverDB(path string) error {
+	stor, err := leveldbstorage.OpenFile(path, false)
+	if err != nil {
+		return err
+	}
+
+	db, err := leveldb.Recover(stor, nil)
+	if err != nil {
+		return err
+	}
+
+	db.Close()
+	stor.Close()
+
+	return nil
+}