When clearing a tree clear its stats too

When a tree is cleared (erased from disk) the corresponding build
status should also be reset.  That is, the `riak-admin search
aae-status` command should display `--` for the last time the tree was
built since it is _not_ built.
Merge pull request #405 from basho/rz/clear-tree-stats

When clearing a tree clear its stats too

Reviewed-by: coderoshi