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

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.