lib/kv: add key-value database api #5587

Add bolt-based key-value database support.

Quick API description:
https://github.com/rclone/rclone/pull/5587#issuecomment-942174768

lib/kv/unsupported.go

@@ -0,0 +1,40 @@
+//go:build plan9 || js
+// +build plan9 js
+
+package kv
+
+import (
+	"context"
+
+	"github.com/rclone/rclone/fs"
+)
+
+// DB represents a key-value database
+type DB struct{}
+
+// Supported returns true on supported OSes
+func Supported() bool { return false }
+
+// Start a key-value database
+func Start(ctx context.Context, facility string, f fs.Fs) (*DB, error) {
+	return nil, ErrUnsupported
+}
+
+// Get returns database for given filesystem and facility
+func Get(f fs.Fs, facility string) *DB { return nil }
+
+// Path returns database path
+func (*DB) Path() string { return "UNSUPPORTED" }
+
+// Do submits a key-value request and waits for results
+func (*DB) Do(write bool, op Op) error {
+	return ErrUnsupported
+}
+
+// Stop a database loop, optionally removing the file
+func (*DB) Stop(remove bool) error {
+	return ErrUnsupported
+}
+
+// Exit stops all databases
+func Exit() {}