A superuser can not manage grants on not owned objects

Implement functions which change their effective
user id ot the one of the definer during execution.

Fixes compatibility with third-party modules.

This patch also adds argument checking to all socket methods.

1. snapshot_save calls snapshot_space
2. snapshot_space uses index->initIterator()
3. initIterator() doesn't work until endBuild() is called
4. index->endBuild() is called by space_end_recover_snapshot()
   *after* snapshot_save()
=> Replica doesn't save some tuples into snapshot on JOIN

This patch changes space_end_recover_snapshot() order.

(fix httpd log messages)

Recovery now needs to be run in a non-sched fiber.
Create a fiber for it in on_bind callback.

On another token:

- rename snap_daemon to snapshot_daemon
- don't output redundant snap_daemon prefix in the log, it's already
in the fiber name.
- miscallenous cleanups (roles).

Test. Fixed in gh-438.

* Rename snap_period and snap_count to snapshot_period, snapshot_count.
* Move snap_daemon to box.internal