Fully validate configuration before reloading

Currently when a config reload request is issued to an agent it will parse and do basic validation of the configuration. There are extra validation checks performed after this and after the previous local state(services, checks etc.) has been unloaded that can still fail and cause the agent to be left in a partially configured state.

Many of those extra checks are not runtime errors but could be validated against the static configuration.

#### Feature Description

Ensure that outside of random runtime issues that we cannot catch ahead of time (out of memory, network failures etc.) that a reload will not leave the Consul agent in an inconsistent state. This could involve creating a secondary state from the first and then swapping once we know the new state is okay or just moving many more of the validation checks to before things get unloaded.

#### Use Case(s)

When you want to reload an agents configuration.