Merge "for each authenticator" algorithm step into "while timer not e…

…xpired" step

This merges the previous step 19 of makeCredential in as a switch case
of step 20, and the previous step 18 of getAssertion in as a switch case
of step 19. This way there is only one step in each algorithm that tries
to express things to do asynchronously for the duration of the timer.

The inline `Issue:`s mentioning underspecified behaviour are replaced
with descriptions of an abstract "set of presently available
authenticators" and `Note:`s indicating that this is intentionally
underspecified and meant to represent different connection and discovery
mechanisms all in one.

This also swaps the order of the previous steps 17 ("Start
|lifetimeTimer|.") and 18 ("Let |issuedRequests| be a new ordered set")
of makeCredential, for consistency with getAssertion and so that the
step "Start |lifetimeTimer|." immediately precedes the step "While
|lifetimeTimer| has not expired [...]".