Fixes a bug whereby servers present in multiple network areas would be
properly segmented in the Router, but not in the gRPC mirror. This would
lead servers in the current datacenter leaving from a network area
(possibly during the network area's removal) from deleting their own
records that still exist in the standard WAN area.
The gRPC client stack uses the gRPC server tracker to execute all RPCs,
even those targeting members of the current datacenter (which is unlike
the net/rpc stack which has a bypass mechanism).
This would manifest as a gRPC method call never opening a socket because
it would block forever waiting for the current datacenter's pool of
servers to be non-empty.
This change attempts to make the delay logic more obvious by:
* remove indirection, inline a bunch of function calls
* move all the code and constants next to each other
* replace the two constant values with a single value
* reword the comments.
The router.Manager is already rebalancing servers for other connection pools, so it can call into our resolver to do the same.
This change allows us to remove the serf dependency from resolverBuilder, and remove Datacenter from the config.
Also revert the change to refreshServerRebalanceTimer
It only needs to be refereced from the Router, because there is only 1 instance, and the
Router can call AddServer/RemoveServer like it does on the Manager.
When calling `GetDatacentersByDistance()` or `GetDatacentersMap()`, an
incorrect condition was used to diplay log message, thus flooding
Consul's logs.
Example of message:
```
[WARN] agent.router: Non-server in server-only area: non_server=myClientNode area=lan
```
This message is only valid for WAN areas, filter to avoid creating
hundreds of logs/s on our clusters, each time someone is calling this
method.
Our logs were flooded by such messages when migrating our Consul servers
from 1.7.7 to 1.8.4.
This will issue fix#8663
* changes some functions to return data instead of modifying pointer
arguments
* renames globalRPC() to keyringRPCs() to make its purpose more clear
* restructures KeyringOperation() to make it more understandable
