Add `delete_device` support

### abstract
This patch provides cmd to delete device permanently in `MogileFS::Worker::Query`.

Client-side usage is like following.

    my $admin = MogileFS::Admin->new(hosts => ["ip:port"])
    $admin->{backend}
          ->do_request(
              "delete_device", 
              {devid => $devid, host => $hostname}
          );

### implementation detail
Actually. this command keeps the primary key `devid` of specified device in TABLE `device`
that prevents reuse of identical `devid`

It moves the `hostid` of specified device to be `0` when `host` id starts from `1`.
Thus, hosts contains no devices can be removed permanently as well.

This patch also prevents any further interactions with deleted devices (`hostid = 0`)
That is, get_all_devices() returns devices with condition `hostid > 0`.

It also validates the specified device contains no file, accepts no incoming file, and is marked as dead,
which prevents deleting of incident and implosion of the cluster.