trie: Add support for nested storage transactions

[dimartiro]

Task summary

With our current implementation we are not supporting nested transactions, we are only supporting 1 level.

Our implementation keeps track of one oldTrie to rollback the current transaction but that does not allow us to use nested transactions

gossamer/lib/runtime/storage/trie.go

Lines 50 to 55 in 8b93d5e

	func (s *TrieState) BeginStorageTransaction() {
	s.lock.Lock()
	defer s.lock.Unlock()
	s.oldTrie = s.t
	s.t = s.t.Snapshot()
	}

Specification reference

The spec says that the function ext_storage_start_transaction should create a new nested transaction

Other information and links

• Substrate implementation

• Kagome implementation