Rename EphemeralFileNode to SessionFileNode.

From the DataTree point of view there's no ephemeral file, it's the
Pulsed's responsibility to maintain which sessions are online which
are offline and explicitly delete all the nodes of a session who is
offline.
Implement session file in DataTree.

- User can create a file of a given session.
- User can delete all the files of a given session.
Pass Pulsed instead of DataTree to command.

- Also add PulsedSessionsHandler servlet for session management.
Implement creation and renew of session (Not ephemeral file).

- Session file (like /pulsed/sessions/0000000000...) is also ephemeral
file.
- Initially the session belongs to server who created it.
- If clients send pulse to other servers then other servers can take
the ownership of the session.
Add SessionFilter to reject requests with expired/unexistent sessions

- Also add inheritance relationship between servlets.
Add global version to each response
Merge pull request #63 from EasonLiao/ephemeral

Implement session feature