Leaving fingerprinters in case that interface changes with plugins.

* Stopping an alloc is implemented via Updates but update hooks are
  *not* run.
* Destroying an alloc is a best effort cleanup.
* AllocRunner destroy hooks implemented.
* Disk migration and blocking on a previous allocation exiting moved to
  its own package to avoid cycles. Now only depends on alloc broadcaster
  instead of also using a waitch.
* AllocBroadcaster now only drops stale allocations and always keeps the
  latest version.
* Made AllocDir safe for concurrent use

Lots of internal contexts that are currently unused. Unsure if they
should be used or removed.

Saves a tiny bit of cpu and some IO. Sadly doesn't prevent all IO on
duplicate writes as the transactions are still created and committed.

$ go test -bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/hashicorp/nomad/helper/boltdd
BenchmarkWriteDeduplication_On-4             500           4059591 ns/op           23736 B/op         56 allocs/op
BenchmarkWriteDeduplication_Off-4            300           4115319 ns/op           25942 B/op         55 allocs/op

Test that snapshot errors don't return a valid tar currently fails.

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.

GC much more aggressively by triggering GCs when allocations become
terminal as well as after new allocations are added.

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.

This PR adds a version package and consolidates version strings into a
Version struct.

Fixup some TODOs and formatting left from new prevAllocWatcher code.

interface has 3 implementations:

1. local for blocking and moving data locally
2. remote for blocking and moving data from another node
3. noop for allocs that don't need to block

This PR unmounts directories from tasks when the alloc is terminal
rather than when it is garbage collected.

/cc @angrycub

This PR does the following:
* Adds a mechanism to emit events in the TaskRunner
* Vendors a new version of Consul-Template that allows extraction of
missing dependencies
* Adds logic to our consul_template.go to determine missing events and
emit them in a batched fashion.
* Refactors the consul_template code to split the run method and take in
a config struct rather than many parameters.

Fixes https://github.com/hashicorp/nomad/issues/2578