Concurrency for generic package publish API

What does this MR do?

The generic package publish API creates the package at first file upload.

When multiple file uploads are initiated by a client in parallel they will each race to be the one also creating the as-of-yet non-existent package record in the database.

Only one such racing request will win and all others will fail with a transaction error because they'll be viewed as attempting to insert duplicate (non-unique) records.

The endpoint must therefore only be called serially to guarantee success of multiple file uploads under a single name and version.

This change adds a documentation note under the pre-requisites section with an attempt at explaining why.

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/290613

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