…#4413

This change allow to rename any well behaving recent agent with an
ID to be renamed safely, ie: without taking the name of another one
with case insensitive comparison.

Deprecated behaviour warning
----------------------------

Due to asceding compatibility, it is still possible however to
"take" the name of another name by not providing any ID.

Note that when not providing any ID, it is possible to have 2 nodes
having similar names with case differences, ie: myNode and mynode
which might lead to DB corruption on Consul server side and
lead to server not properly restarting.

See hashicorp#3983 and hashicorp#4399 for Context about this change.

Disabling registration of nodes without IDs as specified in hashicorp#4414
should probably be the way to go eventually.

block since it breaks the test TestAgentAntiEntropy_Services

While the else case is probably legit, it will be fixed with hashicorp#4414 in
a later release.

enforce this test only for nodes having IDs.

Thus most tests without any ID will work, and allows us fixing

`TestStateStore_EnsureNode` now test registration and renaming with IDs

`TestStateStore_EnsureNodeDeprecated` tests registration without IDs
and tests removing an ID from a node as well as updated a node
without its ID (deprecated behaviour kept for backwards compatibility)

…has no ID

…searching node from its ID

Thus, all tests about renaming were not working properly.

Added the full test cas that allowed me to detect it.

…GetNodeID()

More complete test coverage for GetNodeID

Also fixed comments to be clearer after remarks from @banks