Fixes a deadlock in leadership handling if leadership flapped.

Raft propagates leadership transition to Nomad through a NotifyCh channel.
Raft blocks when writing to this channel, so channel must be buffered or
aggressively consumed[1]. Otherwise, Raft blocks indefinitely in `raft.runLeader`
until the channel is consumed[1] and does not move on to executing follower
related logic (in `raft.runFollower`).

While Raft `runLeader` defer function blocks, raft cannot process any other
raft operations.  For example, `run{Leader|Follower}` methods consume
`raft.applyCh`, and while runLeader defer is blocked, all raft log applications
or config lookup will block indefinitely.

Sadly, `leaderLoop` and `establishLeader` makes few Raft calls!
`establishLeader` attempts to auto-create autopilot/scheduler config [3]; and
`leaderLoop` attempts to check raft configuration [4].  All of these calls occur
without a timeout.

Thus, if leadership flapped quickly while `leaderLoop/establishLeadership` is
invoked and hit any of these Raft calls, Raft handler _deadlock_ forever.

Depending on how many times it flapped and where exactly we get stuck, I suspect
it's possible to get in the following case:

* Agent metrics/stats http and RPC calls hang as they check raft.Configurations
* raft.State remains in Leader state, and server attempts to handle RPC calls
  (e.g. node/alloc updates) and these hang as well

As we create goroutines per RPC call, the number of goroutines grow over time
and may trigger a out of memory errors in addition to missed updates.

[1] https://github.com/hashicorp/raft/blob/d90d6d6bdacf1b35d66940b07be515b074d89e88/config.go#L190-L193
[2] https://github.com/hashicorp/raft/blob/d90d6d6bdacf1b35d66940b07be515b074d89e88/raft.go#L425-L436
[3] https://github.com/hashicorp/nomad/blob/2a89e477465adbe6a88987f0dcb9fe80145d7b2f/nomad/leader.go#L198-L202
[4] https://github.com/hashicorp/nomad/blob/2a89e477465adbe6a88987f0dcb9fe80145d7b2f/nomad/leader.go#L877

Always deliver last value then send close signal.