trie: no need to store preimage if not enabled

[jsvisa]

As the preimage will only be stored if t.preimages != nil, so no need to save them into local cache if not enabled, ref

go-ethereum/trie/secure_trie.go

Lines 265 to 267 in 959f00d

	if t.preimages != nil {
	t.preimages.InsertPreimage(t.secKeyCache)
	}

This will reduce the memory wasted to copy the bytes

[rjl493456442]

Unfortunately, the t.preimage will always be non-nil, as triedb does support the preimage interface.

We need to find an alternative approach to avoid caching preimages in the trie.

[jsvisa]

Unfortunately, the t.preimage will always be non-nil, as triedb does support the preimage interface.

just found the preimage was checked by type assert, and triedb always implement the preimageStore interface, so add another method PreimageEnabled() to explicitly indicate preimage is enabled or not.