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

[sffc]

I'm hitting this error in ZonedDateTimeFormatter FFI, #6233

It gets triggered because I first load the names into the DateTimeNames with the helper functions and then call try_into_formatter which calls load_for_pattern which tries loading the same names again.

[sffc]

I worked around the case hit in #6233 by relaxing the check for is_none(). If all three loads return None, then we can assume that all of the payloads were cached.

The bug in the OP is still reachable, I think.