Undo up to the LIB does not work

[sduchesneau]

2024-02-19T14:54:30.097Z ERRO (sink-kv) buffer undo block: cannot undo down to last valid Block #249162326 (5uaFaxM8JENWi38Kf6xdWcSK1B56YmQ69ccepuDqFYeS) because we already sent you Block #249162348 (tSHqPkLcDMtXRVQu3U7KULAWTsZer4PA9miQGLqKcCz) which is after last valid block

The kv sink should handle undoing up to the lib, this is probably an off-by-one error.

Also, the error message is confusing.
"Cannot undo down to last valid block 249162326" -> it is not because we sent block 249162348... presumably, it's because the undo data has been truncated to "above 249162326". fix the error message too!