add create account/user/role (#4741)

1.  Enable multi tenant in default

- add create account statement.
- add create user statement.
- add create role statement.
- with two internal users -- root and dump (both password 111) in sys tenant.
- login format: the '-u' option of mysql client has been extended to "tenant:user:role" or "tenant:user"(public role) or "user"(sys tenant, public role)

2. Already known restrictions in this initial version:

- no privilege check
- no drop/alter statement
- no view
- only tenant isolation
- user can see the whole catalog of the tenant that it is belonged to.
- remove the .store directory when mo boots with multi-tenant verison at first time. Just only once.

3. Remove db-server

Approved by: @reusee, @iamlinjunhong, @aressu1985