Consider dropping error result from ImmutableTree.Iterator

[elias-orijtech]

Right now every caller of ImmutableTree.Iterator must handle its error result, which in turn leads to errors being ignored:

iavl/immutable_tree.go

Line 49 in 64ffc27

t.Iterate(func(key []byte, val []byte) (stop bool) { //nolint:errcheck

However, the only reason an error can be returned is if IsFastCacheEnabled returns an error, which supposedly is very rare.

A better API is to fall back to NewIterator when IsFastCacheEnabled fails and remove the error result. This benefits every caller, direct and indirect.

CC @odeke-em