feat(store): add tail header (#200)

## Overview

Add `tailHeader` as a part of backward sync. Currently it's initialized
to initial header and isn't exported. Harmless addition to simplify
future PRs.

store/store.go

@@ -51,6 +51,8 @@ type Store[H header.Header[H]] struct {
 	writesDn chan struct{}
 	// writeHead maintains the current write head
 	writeHead atomic.Pointer[H]
+	// tailHeader maintains the current tail header.
+	tailHeader atomic.Pointer[H]
 	// pending keeps headers pending to be written in one batch
 	pending *batch[H]
 
@@ -121,6 +123,8 @@ func (s *Store[H]) Init(ctx context.Context, initial H) error {
 		return err
 	}
 
+	s.tailHeader.Store(&initial)
+
 	log.Infow("initialized head", "height", initial.Height(), "hash", initial.Hash())
 	s.heightSub.Pub(initial)
 	return nil

store/store_test.go

@@ -22,6 +22,8 @@ func TestStore(t *testing.T) {
 	ds := sync.MutexWrap(datastore.NewMapDatastore())
 	store := NewTestStore(t, ctx, ds, suite.Head())
 
+	assert.Equal(t, *store.tailHeader.Load(), suite.Head())
+
 	head, err := store.Head(ctx)
 	require.NoError(t, err)
 	assert.EqualValues(t, suite.Head().Hash(), head.Hash())