Clarify rollback attack prevention and fast-forward attack recovery

[lukpueh]

This PR combines and updates #72, which clarifies the verification routines for delegated targets, and #74 and #85, which fix #71, i.e. resolve ambiguity around rotating keys and deleting metadata, in order to recover from fast-forward attacks, by temporarily disabling rollback attack checks.

Pre-requisite

• 0ecf980 reverts Remove problematic targets rollback attack check #65 (see commit message for rationale)

Takeover of #72

• 75ac163 is a verbatim copy of the sole commit from Clarify how delegated roles are downloaded #72 courtesy of @erickt
• 329361a, c36e0c0 and e4a6f9e address Clarify how delegated roles are downloaded #72-reviews (by @lukpueh and @mnm678)

Takeover of #85

• Please see commit messages of b677f6a, fa2d263 and most notably 65e042d for details