Refactor Consul Syncer into new ServiceClient

Fixes #2218

Support for TLSSkipVerify in other checks coming soon!

Previous implementation assumed all struct fields were included in
service and check IDs. Service IDs never include port labels and check
IDs *optionally* include port labels, so lots of things had to change.

Added a really big test to exercise this.

...and still protect against leaking agent entries in Consul on
shutdown.

Previously it applied a stream of operations. Reconciling state is less
complex and error prone at the cost of slightly higher CPU/memory usage.

Fixes #2478 #2474 #1995 #2294

The new client only handles agent and task service advertisement. Server
discovery is mostly unchanged.

The Nomad client agent now handles all Consul operations instead of the
executor handling task related operations. When upgrading from an
earlier version of Nomad existing executors will be told to deregister
from Consul so that the Nomad agent can re-register the task's services
and checks.

Drivers - other than qemu - now support an Exec method for executing
abritrary commands in a task's environment. This is used to implement
script checks.

Interfaces are used extensively to avoid interacting with Consul in
tests that don't assert any Consul related behavior.