See #347

Conflicts:
	doc/www-data.in/download.cmake

Conflicts:
	cmake/rpm.cmake
	extra/rpm/tarantool-scl.rpm.spec
	extra/rpm/tarantool.rpm.spec.in

Add provisioning for user type.
Support user type in alter.
Do not return roles in user_by_name().
Make it possible to grant privileges on roles ('execute').

 * remove vclock_merge
 * remove vclock_destroy
 * use `server` instead of `node`
 * use `signature` instead of `lsnsum`
 * refactor vclock_from_string code

Add access checks in Lua iterators.

Add a owner for a user, use owner privileges to check
whether or not it's possible to drop a user.

Rebuild empty.snap, it can be used as bootstrap.snap to
fill the schema from scratch.

Update bootstrap.snap with an index on _user[2] (second
field in _user).

Conflicts:
	extra/rpm/tarantool.rpm.spec.in

t chec  # On branch stable

Conflicts:
	src/lua/bsdsocket.lua
	src/lua/init.cc
	test/box/bsdsocket.result
	test/box/bsdsocket.test