[Pebble] events need overwrite protection

[AlexHentschel]

Currently, we just write events for a block ID:

flow-go/storage/store/events.go

Line 58 in 22fed8b

err := operation.InsertEvent(writer, blockID, event)

this should be protected by a lock

[zhangchiqing]

Addressed by https://github.com/onflow/flow-go/pull/8005/files#diff-db1f3e68113391168d7f6bf516cfc5b9c2ac5c0d0f106751171f48b4da7d7678R19