Fix Session.Renew to care about 404s properly

[rboyer]

Fixes part of #1040.

Needs some sort of handling for how to deal with RenewPeriodic.

[rboyer]

In a test program with a slightly modified RenewPeriodic() call, I had it print the SessionEntry and error return value every time it was called. I deliberately had the program take a very long sleep in between creating the session and attempting to renew it (it had delete behavior). This was to showcase what happens when the leader evaporates your session while you were trying to be a good renewing citizen (but something like a long GC pause or similar caused you to miss your renewal window):

2015/06/17 09:04:11 created session 85bb5662-4705-2e1b-4b7f-7997c5d2cd55 with ttl of 10s
2015/06/17 09:05:01 renewing session 85bb5662-4705-2e1b-4b7f-7997c5d2cd55
2015/06/17 09:05:01 got entry=[*api.SessionEntry/<nil>], err=[*errors.errorString/Unexpected response code: 404 (Session id '85bb5662-4705-2e1b-4b7f-7997c5d2cd55' not found)]
...this repeats several times...
2015/06/17 09:05:06 session died: 85bb5662-4705-2e1b-4b7f-7997c5d2cd55

It only dies because the full TTL elapsed. With the proposed changes on this MR:

2015/06/17 09:11:17 renewing session d8ecada8-ec39-8336-7eee-847a34d5a2f1
2015/06/17 09:11:17 got entry=[*api.SessionEntry/<nil>], err=[<nil>/<nil>]
2015/06/17 09:11:17 session died: d8ecada8-ec39-8336-7eee-847a34d5a2f1

It doesn't try to keep renewing the session that has evaporated (which is the sane thing to do).

[rboyer]

Do I have to do anything else to help move this along?

[armon]

@rboyer Sorry about the delay. Could you please add a test case?

[rboyer]

Rebased and added tests.

[rboyer]

Does this look sufficient now?

[slackpad]

Sorry been swamped but I'll try to get to this in the next day or so.

[slackpad]

LGTM - thanks!