Prevent status flap when re-registering a check

[ShimmerGlass]

Fixes point #2 of: #4903

When registering a service each healthcheck status is saved and restored (https://github.com/hashicorp/consul/blob/master/agent/agent.go#L1914) to avoid unnecessary flaps in health state.
This change extends this feature to single check registration by moving this protection in AddCheck() so that both PUT /v1/agent/service/register and PUT /v1/agent/check/register behave in the same idempotent way.

Steps to reproduce

1. Register a check :

curl -X PUT \
  http://127.0.0.1:8500/v1/agent/check/register \
  -H 'Content-Type: application/json' \
  -d '{
  "Name": "my_check",
  "ServiceID": "srv",
  "Interval": "10s",
  "Args": ["true"]
}'

2. The check will initialize and change to passing
3. Run the same request again
4. The check status will quickly go from critical to passing (the delay for this transission is determined by https://github.com/hashicorp/consul/blob/master/agent/checks/check.go#L95)

[pierresouchay]

LGTM

[pierresouchay]

@banks while #4905 is harder to merge, this one is simple enough and just improve the situation by not resetting the state and avoid unnecessary flap of services, would be a nice improvement IMHO

[banks]

Thanks @Aestek @pierresouchay, I've added it to the list to consider first glance seems good.

To make sure I understand, The code that is removed here already means that PUT /v1/agent/service/register behaves correctly right? It's only the PUT /v1/agent/check/register that exhibits the flap bug?

[ShimmerGlass]

@banks yes, /v1/agent/service/register was already protected against flaps, this change extends the protection to /v1/agent/check/register

[mkeeler]

Looks good to me.

[pierresouchay]

@mkeeler thank you