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/database.go

@@ -0,0 +1,69 @@
+// Package database provides KV database for meta-information
+package database
+
+import "errors"
+
+// Errors for Storage
+var (
+	ErrNotFound = errors.New("key not found")
+)
+
+// StorageProcessor is a function to process one single storage entry
+type StorageProcessor func(key []byte, value []byte) error
+
+// Reader provides KV read calls
+type Reader interface {
+	Get(key []byte) ([]byte, error)
+}
+
+// PrefixReader provides prefixed operations
+type PrefixReader interface {
+	HasPrefix(prefix []byte) bool
+	ProcessByPrefix(prefix []byte, proc StorageProcessor) error
+	KeysByPrefix(prefix []byte) [][]byte
+	FetchByPrefix(prefix []byte) [][]byte
+}
+
+// Writer provides KV update/delete calls
+type Writer interface {
+	Put(key []byte, value []byte) error
+	Delete(key []byte) error
+}
+
+// Storage is an interface to KV storage
+type Storage interface {
+	Reader
+	Writer
+
+	PrefixReader
+
+	CreateBatch() Batch
+	OpenTransaction() (Transaction, error)
+
+	CreateTemporary() (Storage, error)
+
+	Open() error
+	Close() error
+	CompactDB() error
+	Drop() error
+}
+
+// Batch provides a way to pack many writes.
+type Batch interface {
+	Writer
+
+	// Write closes batch and send accumulated writes to the database
+	Write() error
+}
+
+// Transaction provides isolated atomic way to perform updates.
+//
+// Transactions might be expensive.
+// Transaction should always finish with either Discard() or Commit()
+type Transaction interface {
+	Reader
+	Writer
+
+	Commit() error
+	Discard()
+}