Mark the server as dead when it is disposed

When the server is disposed, mark as dead so that an in-progress operation
or view request will not select it. This should eliminate some ODE's but
not all. This commit also makes the _disposed and _isDead variables
volatile.

Change-Id: I88b2e12079582c66725d898a6eaeee154163ea99
Reviewed-on: http://review.couchbase.org/42450
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>