[CORE] Delete shard content under lock

Once we delete the the index on a node we are closing all resources
and subsequently need to delete all shards contents from disk. Yet
this happens today under a lock (the shard lock) that needs to be
acquried in order to execute any operation on the shards data
path. We try to delete all the index meta-data once we acquired
all the shard lock but this operation can run into a timeout which causes
the index to remain on disk. Further, all shard data will be left on
disk if the timeout is reached.

This commit removes all the shards data just before the shard lock
is release as the last operation on a shard that belongs to a deleted
index.

supersedes #8608 & relates to #9009 
Isn't this redundant since the exists check just returned false?