missing interaction with driver to restore

probably need to abandon this and test at client layer

Fixes a panic. Left a comment on how the behavior could be improved, but
this is what releases <0.9.0 did.

* Migrated all of the old leader task tests and got them passing
* Refactor and consolidate task killing code in AR to always kill leader
  tasks first
* Fixed lots of issues with state restoring
* Fixed deadlock in AR.Destroy if AR.Run had never been called
* Added a new in memory statedb for testing

Update comments and address some PR comments from #4775

Do not use cty in drivers

drivers/exec: add exec implementation

UI: Make the global logo link to the jobs page (home page)

AllocRunner v2 Feature Branch PR

was causing typed nil interface panics and served no purpose

Was mock_driver before plugins, so keep the name.

from #4777 comments

Do not register mock_driver on release builds.

PR #4392 was merged to master *after* allocrunnerv2 was branched, so the
client-specific portions must be ported from master to arv2.

Now passing the TaskDir struct to prestart hooks instead of just the
root task dir itself as dispatch needs local/.