This commit reverts most of dea940c7 and d89c7024. The "fix" is to run
attach fairings on a new thread. If a runtime is already running, it is
used. Otherwise, the future is executed in a single-threaded executor.
This is largely an internal change. Prior to this commit, the 'Manifest'
type, now replaced with the 'Cargo' type, robbed responsibility from the
core 'Rocket' type. This new construction restores the previous
responsibility and makes it clear that 'Cargo' is _only_ for freezing,
and representing the stability of, Rocket's internal state.
Sergio Benitez