Consul Session TTLs

The design of the session TTLs is based on the Google Chubby approach
(http://research.google.com/archive/chubby-osdi06.pdf). The Session
struct has an additional TTL field now. This attaches an implicit
heartbeat based failure detector. Tracking of heartbeats is done by
the current leader and not persisted via the Raft log. The implication
of this is during a leader failover, we do not retain the last
heartbeat times.

Similar to Chubby, the TTL represents a lower-bound. Consul promises
not to terminate a session before the TTL has expired, but is allowed
to extend the expiration past it. This enables us to reset the TTL on
a leader failover. The TTL is also extended when the client does a
heartbeat. Like Chubby, this means a TTL is extended on creation,
heartbeat or failover.

Additionally, because we must account for time requests are in transit
and the relative rates of clocks on the clients and servers, Consul
will take the conservative approach of internally multiplying the TTL
by 2x. This helps to compensate for network latency and clock skew
without violating the contract.

Reference: https://docs.google.com/document/d/1Y5-pahLkUaA7Kz4SBU_mehKiyt9yaaUGcBTMZR7lToY/edit?usp=sharing
Added more tests. Also added return of 404 if the session id to renew is not found
Added more tests
Clean up code based on feedback from armon
Took out StateSnapshot SessionListTTL also
Remove hardcoded wait time in session TTL tests
Took out usage of snapshot SessionListTTL
Fixed clearSessionTimer, created invalidateSession, added invalid TTL test
Add invalidateSession test
Merge pull request #524 from amalaviy/session_ttl

Consul Session TTLs