don't retry entry guards if they're bridges without descriptors

When we don't yet have a descriptor for one of our bridges, disable
the entry guard retry schedule on that bridge. The entry guard retry
schedule and the bridge descriptor retry schedule can conflict,
e.g. where we mark a bridge as "maybe up" yet we don't try to fetch
its descriptor yet, leading Tor to wait (refusing to do anything)
until it becomes time to fetch the descriptor.

Fixes bug 40497; bugfix on 0.3.0.3-alpha.
9 jobs for bug40396 in 18 minutes and 25 seconds (queued for 4 seconds)
latest
Status Name Job ID Coverage
  Test
passed debian-disable-dirauth #46750
amd64

00:13:24

passed debian-disable-relay #46751
amd64

00:13:24

passed debian-distcheck #46746
amd64

00:14:29

passed debian-docs #46747
amd64

00:01:16

passed debian-hardened #46745
amd64

00:14:53

passed debian-integration #46748
amd64

00:18:24

passed debian-minimal #46744
amd64

00:12:09

passed debian-nss #46752
amd64

00:12:30

passed debian-tracing #46749
amd64

00:15:02