Close #716. Fix #702. don't panic when dropping non-existent shard

The server would crash if you attempted to drop a shard multiple times.
It would also cause a problem on restart if a shard was dropped, but the
server restarted before a raft log compaction.
Update the change log

[skip ci]
cast *C.value_array from *C.struct___0 #699
Merge pull request #723 from chobie/fix-669-cast-to-value_array

cast *C.value_array from *C.struct___0 #699
Merge branch 'master' of https://github.com/rabdureh/influxdb