Refactor a AppStopper which can be used in background jobs.

Inline methods in DeaClient

[#60026682]

Signed-off-by: Phan Le <ple@pivotallabs.com>
Start message bus in a thread so that the DelayedJob main thread does not starve EventMachine

Signed-off-by: Phan Le <ple@pivotallabs.com>