Heartbeat timeout means the period length that indicates heartbeat is
out of service, which is different from heartbeat interval.
So we should use '-peer-heartbeat-interval' instead of
'-peer-heartbeat-timeout' in etcd.
'-peer-heartbeat-timeout' is deprecated but still could be used.
The boot order for peers is -discovery, -peers, log data, forming
new cluster itself.
Special rules:
1. If discovery succeeds, it would find peers specified by discover URL
only.
2. Etcd would fail when meeting bad -discovery, no -peers and log data.
Add TestDiscoveryDownNoBackupPeersWithDataDir as the test.
This is an initial version of the bootstrap code that seems to work
under the normal circumstances. I need to mock out a server that will
test out all of the error cases now.
Refactor config into its own package. Trying to tease the config from
the server so that all of the control surfaces are exposed in the Server
for easier testing.
