Fix checksum validation in datetime time zone names

[sffc]

Currently we check the checksum only on the first load of a time zone name payload. However, we should retain the checksum in case we need to do further loads, at least the one in mz_periods.

This bug can probably be triggered by making a formatter with the pattern "ZZZZ VVVV" (two different time zone fields that both require loading metazone periods).

Also, consider optimizing BlobSchema::get_checksum to use the probe function. We shouldn't need to binary-search the ZeroTrie.

#6046