Suppress "AcceptToMemoryPool: joinsplit requirements not met" during IBD

[str4d]

During InitialBlockDownload (e.g. when reindexing), transactions that spend Notes cannot be validated because the commitment tree corresponding to their anchor cannot be located. This correctly results in transactions that spend Notes being rejected from the node's mempool, but creates a lot of noise in the debug.log. We should suppress this error during that time, or at the very least downgrade it from an error to a warning or informational message.