Fix zallet zcashd migration strict mode

[Zk-nd3r]

Summary

• pass the inverted allow_warnings flag into ZcashdDump::from_bdb_dump so the default migration path is strict in the first parser stage
• keeps the later parse_dump strictness unchanged

Testing

• cargo fmt --check
• cargo check -p zallet (fails in this Windows environment before checking this crate because dlltool.exe is missing for the GNU toolchain)

Fixes #444