leveldb iterator should be deleted before the db is deleted

[yzhliu]

In some old version of leveldb, the iterator must be deleted before the db. Otherwise, it will cause a failure like:

compute_image_mean: version_set.cc:715: leveldb::VersionSet::~VersionSet(): Assertion `dummy_versions_.next_ == &dummy_versions_' failed.
./compute-mean.sh: line 2: 26811 Aborted ./build/tools/compute_image_mean train_leveldb mean.binaryproto leveldb

This is what I got on centos-6.5, rhel-6.5, with epel6 - leveldb-1.7.0-2.el6
Someone else had encountered the same problem, see https://groups.google.com/forum/#!topic/leveldb/I0_gN82a3vI

[shelhamer]

@Javelinjs can you check if this is addressed by #1748? If so this will be fixed once that is merged (soon).

[longjon]

Should work fine in #1748 on account of automatic destructor order, but let me know if you think otherwise @Javelinjs.

[yzhliu]

@shelhamer @longjon I think it works, just ignore this PR, thanks.

[shelhamer]

Closing as fixed by #1748. Thanks for submitting a patch and checking @Javelinjs.