Fixes #3561

Fixes the panic mentioned in
https://github.com/hashicorp/nomad/issues/3420#issuecomment-341666932

While a leader task dying serially stops all follower tasks, the
synchronizing of state is asynchrnous. Nomad can shutdown before all
follower tasks have updated their state to dead thus saving the state
necessary to hit this panic: *have a non-terminal alloc with a dead
leader.*

The actual fix is a simple nil check to not assume non-terminal allocs
leader's have a TaskRunner.

* Allow server TLS configuration to be reloaded via SIGHUP

* dynamic tls reloading for nomad agents

* code cleanup and refactoring

* ensure keyloader is initialized, add comments

* allow downgrading from TLS

* initalize keyloader if necessary

* integration test for tls reload

* fix up test to assert success on reloaded TLS configuration

* failure in loading a new TLS config should remain at current

Reload only the config if agent is already using TLS

* reload agent configuration before specific server/client

lock keyloader before loading/caching a new certificate

* introduce a get-or-set method for keyloader

* fixups from code review

* fix up linting errors

* fixups from code review

* add lock for config updates; improve copy of tls config

* GetCertificate only reloads certificates dynamically for the server

* config updates/copies should be on agent

* improve http integration test

* simplify agent reloading storing a local copy of config

* reuse the same keyloader when reloading

* Test that server and client get reloaded but keep keyloader

* Keyloader exposes GetClientCertificate as well for outgoing connections

* Fix spelling

* correct changelog style