Get session from socket._meteorSession

`socket._meteorSession.connectionHandle` has the session, no need to use
a temporary handler.
Fix memory leak

Memory usage can go high if there is a large number of concurrent
sessions.

 - remove sessionMap
 - store last active time on session object
 - update active time inside `processMessage`
Refactor

 - create method `handleSessionActivity` in system Moodel
 - place `if(Kadira.connected)` after calling original method
Refactor handleSessionActivity

Use this function to handle all session activity.
Fix tests
Revert "published for meteor 1.0"

This reverts commit 98c8b4dd27fd4e3514df03ee2a066ca866cce8b8.
Do not track `unsub`
Remove unnecessary `_isReconnect` flag
No need to runInFiber
Merge remote-tracking branch 'thanish/memory-leak-fix'
version bumped