Vendor go.etcd.io/bbolt @ v1.3.1-etcd.8

Signed-off-by: John Howard <jhoward@microsoft.com>

This is the maintained version of boltdb, which includes the Windows-specific fix
detailed in https://github.com/etcd-io/bbolt/pull/122

vendor/go.etcd.io/bbolt/boltsync_unix.go

@@ -0,0 +1,8 @@
+// +build !windows,!plan9,!linux,!openbsd
+
+package bbolt
+
+// fdatasync flushes written data to a file descriptor.
+func fdatasync(db *DB) error {
+	return db.file.Sync()
+}