Rejigger the governor implementation

Previously we'd spawn a number of processes to send messages to
'noconnect' / 'nosuspend' nodes.  Now we're buffering the messages that
need to be sent directly in each governor and sending them one at a
time.  This prevents the net_kernel from tipping over in the noconnect
case.  We also decide whether to drop messages based on memory
consumption in the node instead of process limits (since we're not
spawning anymore).

BugzID: 23717
BugzID: 23718
Start a supervised rexi_governor per node

This generalizes rexi_server_mon to start per-node versions of a server
specified by a child module.

BugzID: 23717
BugzID: 23718
Remove gov_manager from supervision

We'll remove the module itself in a subsequent release.
Try to start per-node servers immediately

I can't think of a good reason not to do this.
Don't block the governor to send a message

This allows the governor to continue prioritizing incoming requests
instead of hanging for several seconds to try to connect to the remote
node.

BugzID: 23717
BugzID: 23718
Rename governor to buffer

Buffer describes the behavior quite a bit better than governor.

BugzId: 23717
BugzId: 23718
Customize the upgrade instructions for 2.3 -> 2.4

BugzID: 23717
BugzID: 23718
Merge pull request #18 from cloudant/23717-governor-overload

BugzID: 23717
BugzID: 23718