exports: Add a reference count to export_item_t structs and ensure they are correctly used

Summary:
This diff fixes a bug in the NFS daemon where the auth cache would use an export item after it was free'd by the
auth params refresh thread. This usually manifests as a crash in production, when exports files are updated by chef.

Since each auth cache entry holds a pointer to an export_item_t it makes sense that it should first get a reference to it.

Freein'g the export_item_t struct happens only in `exp_item_unref()`, once the reference count has dropped to 0.

This diff also fixes a use-after-free bug in the auth-cache, in the insertion path.

In _cache_item(), if we find an entry in the dict, we update that entry with a timestamp & ref the export item associated with it.
However, if the item already existed and we called old_cache_insert() with the same key, we gave the dict permission to free the old entry.
We then end up using that entry.

The fix is to use dict_set_static_bin() instead of dict_set_bin() which informs the dict that the pointer we are giving it belongs to us.

This is a port of D5780476, D5785038 to 3.8

Change-Id: I5cdcdc1cc6abad26c7077d66a14f263da07678ac
Reviewed-on: https://review.gluster.org/18248
Reviewed-by: Shreyas Siravara <sshreyas@fb.com>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Smoke: Gluster Build System <jenkins@build.gluster.org>

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