Issue #8791 : Harden the Mask fields database mapping store

[bamaer]

Store an HMAC of each source value when a pattern has a hash secret, and move plain-text rows to their hashed key. Reserve sequence numbers with a row lock and add a unique key on the masked value, so two processes never hand out the same value. Share a mapping store by resolved URL and user. Clear the failed and later fields of an error row. Add trim and ignore-case options and key numbers by value. Stop when a hash secret does not resolve.

fixes #8791

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