mod_storage_appendmap: Fix keyvalue writes store_raw() takes a filename, unlike most other datamanager functions that take a series of path components, leading to this bug where it would write the hostname into a file named after the first argument (commonly a username) in the working directory.

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summary: Storage benchmark tool
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To benchmark `mod_storage_internal`:

    prosodyctl mod_benchmark_storage internal

Does a number (aprox. 10000) of reads and writes and to the selected
storage driver and tells you how long it took.