Enforce the limits returned by /info/configuration about syncstorage-rs HOT 3 CLOSED

Please be careful here WRT off by one errors. The clients expect MAX to be the highest valid value of these (see mozilla-services/server-syncstorage#74 and mozilla-services/server-syncstorage#73).

If it helps at all, the Desktop JS code is fairly well commented in terms of explaining what the client expects from the limits.

(And yeah, I don't think there's any way you can enforce the _BYTES limits except for manually, since they refer to a subset of the bytes in the requests)

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#104 began enforcing all these limits except the 2 batch settings max_total_{bytes/records}

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max_total_{bytes/records} are now checked as of #108.

Logged a somewhat related #114 for further enhancements from the go version.

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