Fix data races in HA when slaves return in the pool.

Backport of https://github.com/xapi-project/xen-api/pull/1387 for Boston-lcm.

In HA, during the recovery process of failed hosts, there were data races due to the conflicts between the liveness information of a returning host and its live status as determined by the xha daemon.
These commits are fixing all the problems found when Xapi_hooks.host_post_declare_dead, which is called on every dead hosts, took few minutes to complete, leaving a large window to the dead hosts to come back in the pool.