Bug 2194: Find primary shard on remote ShardManager

Bug 2194: Find primary shard on remote ShardManager

If there is no local shard, the ShardManager now forwards the FindPrimary
message to another ShardManager member.

I changed the FindPrimary and PrimaryFound messages to Serializable.
Previously they implemented SerializableMessage but they didn't have
equivalent protobuff messages so they couldn't be sent over the wire.
These are simple messages and we don't need protobuff.

I also obsoleted the ActorNotInitialized and PrimaryNotFound messages as
we also have equivalent exception classes (which are inhently
Serializable) so the former messages are redundant. This avoids
translation in ActorContext#findPrimaryShardAsync.

Change-Id: I5762ec1dafb9aa12ee8230efe245b154b0bb9b72
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>