Make Definitions a top-level section.

This makes it clear that the Definitions are not non-normative like the Introduction.
Bug 27268 - Add a definition of a distinctive identifier
Bug 27268, 25923 - Replace persistentUniqueIdentifier with distinctiveIdentifier.

Also, replace other instances of "unique identifier" with Distinctive Identifier.
Bug 25923 - Enforce distinctiveIdentifier in the Queue a "message" Event algorithm.
Bug 27272 - Normatively require distinctive identifiers to be encrypted on the Key System level

Adds a new Implementation Requirements section.
Recommend avoiding use of Distinctive Identifiers when possible.
Bug 27166 - All identifiers associated with a user should be clearable in the same way cookies are