It should be performed before we're able to process blocks or before we
shutdown (which uses addition lock), fix
panic: assignment to entry in nil map
goroutine 5755 [running]:
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemoryStore).drop(...)
/home/rik/dev/neo-go/pkg/core/storage/memory_store.go:74
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemoryStore).PutChangeSet(0xc00093e700, 0xc0002051a0, 0xc000205200, 0x1, 0x1)
/home/rik/dev/neo-go/pkg/core/storage/memory_store.go:100 +0x25c
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemoryStore).PutBatch(...)
/home/rik/dev/neo-go/pkg/core/storage/memory_store.go:90
github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).removeOldStorageItems(0xc000206a00)
/home/rik/dev/neo-go/pkg/core/blockchain.go:495 +0x33a
created by github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).jumpToStateInternal
/home/rik/dev/neo-go/pkg/core/blockchain.go:553 +0x68a
FAIL github.com/nspcc-dev/neo-go/pkg/core 52.084s
294a70646e