database: Store mail filename as a new 'direntry' term, not as 'data'.

Instead of storing the complete message filename in the data portion
of a mail document we now store a 'direntry' term that contains the
document ID of a directory document and also the basename of the
message filename within that directory. This will allow us to easily
store multple filenames for a single message, and will also allow us
to find mail documents for files that previously existed in a
directory but that have since been deleted.

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