core-io marshaller should take the locale from the client header

Since NXP-19017 the locale is taken from the user pref then from the request header.
This is a performance problem because each request is going to access the user pref document to get the locale.

Client should be responsible for sending the locale it's interested in.
There can be an additional REST API to get the preferred server-side user's locale but it should be only called once by the client then cached.
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The client is now responsible for sending the locale it is interested in.