libcore is a thick a thick bundle with almost everything.
Rename it to libserver.

Extract the minimal server core with fibers & exceptions into libcore.

Add a prototype unit test for fibers.

Avoid possible trouble with out-of-memory when duplicating xdir path.
Make a few more constants static.

xlog is either a write ahead log or snapshot.
Use a common short prefix for all functions.

Conflicts:
	src/box/CMakeLists.txt
	src/ffisyms.cc
	src/lib/CMakeLists.txt
	src/lib/salad/CMakeLists.txt
	src/lua/digest.lua
	test/box/digest.test
	test/unit/CMakeLists.txt

Update tests to use this role.

Check ER_GRANT error.

Check distinct sources of privileges.

Test the session cache of global access.
Test lack of grant option.

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

Test revoke a privilege from a role using a different grant path.