chore: rename 'heartbeat timeout' to 'heartbeat interval'

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.
Merge pull request #577 from unihorn/6

chore: rename 'heartbeat timeout' to 'heartbeat interval'
Add links to haskell bindings and scrz project
feat(Documentation): add a section on consistent reads

We have waited too long to document `consistent=true`. Document it!
Merge pull request #585 from philips/document-consistent

feat(Documentation): add a section on consistent reads
Merge pull request #586 from wereHamster/patch-1

docs: Add links to haskell bindings and scrz project