refactor server process(), clarifying code and fixing bugs

[rjbs]

• Eliminate bogus metrics right up front. Warn and drop them.
• Rename variables: data, fields, bits, bit... lots of overly generic names. Instead, produce four clear variables: key, value, mtype, and rest.
• Break the big if/else's clauses into distinct functions to be easier to read on their own.
• Avoid some type mismatches. Previously, an Int could end up in what would otherwise be a List of Str.

[sivy]

Will this break any current installations? If not, I'll go ahead and merge...

[rjbs]

I should be safe to merge. It should only change implementation, not behavior, other than catching a few type errors earlier.