RPC Timeout/Retries account for blocking requests

[joel0]

I've ported the Nomad fixes from hashicorp/nomad#8921 and hashicorp/nomad#9064.

The only major differences:

• Consul has a configurable MaxQueryTime and DefaultQueryTime, which is a const in Nomad.
• agent/consul/rpc.go:ForwardRPC() didn't start the firstCheck timer until after the first call. I did not see any motivation for it to be different than agent/consul/client.go:RPC() and Nomad's client/rpc.go:RPC(), so I moved it to the top of the function.

[joel0]

I see that I broke some tests with this. I'm working on fixing those.

I have pushed a fix that moves the timeout check to avoid retrying past the timeout. The tests appear to be passing on my machine*.

^{*I could not get make test to work. Using gotestsum -- -p 2 ./agent returned 3132 tests passed and 1 skipped in 91s.}