main: retrieve config from env vars
main: add test for setting flags
wal: OpenFromIndex fails if it cannot find previous index

Example:
We save entry 1, 2, 3 to WAL.
If we try to open 100, it should fail.
Merge pull request #1040 from jonboulle/1040_config_env_vars

etcd needs to be configurable through env vars
etcdserver: use simple PRNG for GenID
wal: add ErrFileNotFound and ErrIndexNotFound
Merge pull request #1092 from jonboulle/dumb_gen_id

GenID: use a fast prnd
Merge pull request #1099 from unihorn/128

wal: OpenFromIndex fails if it cannot find previous index
etcdserver: add nodeStepRecorder
etcdserver: add forceGosched