BUG 3054 : Check readiness status of datastore when receiving MemberUp.

BUG 3054 : Check readiness status of datastore when receiving MemberUp.

A Follower knows who the Leader is when it receives an AppendEntries message,
however it does record the address of the leader, that is supplied by the
ShardManager when the ShardManager receives a MemberUp. The Follower cannot
be considered ready till it knows both who the leader is and what is it's
address. This patch ensures that the ShardManager counts down the readiness
latch only when the follower knows both.

This also ensures that FindPrimaryShard will only return a primary if the Leader
is truly known.

Change-Id: I1cd6b62124d50c9eca37368d0379b0c75168bb76
Signed-off-by: Moiz Raja <moraja@cisco.com>