[x/sync] Update target locking

[patrick-ogrady]

Why this should be merged

Might resolve: #1760

How this works

We ensure that the sync target cannot be updated while we are determining whether to re-add completed workItems to the queue or mark as processed.

How this was tested

HyperSDK: ava-labs/hypersdk#278

[patrick-ogrady]

May be a simpler fix to just reverse order of lock acquisition here:

avalanchego/x/sync/manager.go

Lines 528 to 539 in 8694274

	func (m *Manager) UpdateSyncTarget(syncTargetRoot ids.ID) error {
	m.workLock.Lock()
	defer m.workLock.Unlock()
	select {
	case <-m.doneChan:
	return ErrAlreadyClosed
	default:
	}
	m.syncTargetLock.Lock()
	defer m.syncTargetLock.Unlock()

Then just hold onto the updateSyncTarget lock (continue assuming workLock is managed by enqueueLock).