Fixes#3380
Adds address_mode to checks (but no auto) and allows services and checks
to set literal port numbers when using address_mode=driver.
This allows SDNs, overlays, etc to advertise internal and host addresses
as well as do checks against either.
Fixes#3342
Two bugs were fixed:
* Closing the StreamFramer's exitCh before setting the error means other
goroutines blocked on exitCh closing could see the error as nil. This
was *not* observered.
* parseFramerError on Windows would fall through and return an
improperly captured nil err variable. There's no need for
parseFramerError to be a closure which fixes the confusion.
* 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
The Client.allocs map now contains all AllocRunners again, not just
un-GC'd AllocRunners. Client.allocs is only pruned when the server GCs
allocs.
Also stops logging "marked for GC" twice.
