check for app id presence in staging message

also make errors consistent (lowercase and extract most into variables)

Signed-off-by: Alex Suraci <asuraci@pivotallabs.com>
Signed-off-by: Phan Le <ple@pivotallabs.com>
bump deps

Signed-off-by: Phan Le <ple@pivotallabs.com>
validate incoming staging requests

also split staging handling into a separate "inbox" package for
symmetry and clarity

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

Signed-off-by: Phan Le <ple@pivotallabs.com>
keep trying to subscribe until successful

after initially subscribing, yagnats will manage the connection and
subscription for us

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