Modify the password and privilege checking logic

[morningman]

Password checking logic

There are some problems with the current password and permission checking logic. For example:
First, we create a user by:
create user cmy@"%" identified by "12345";

And then 'cmy' can login with password '12345' from any hosts.

Second, we create another user by:
create user cmy@"192.168.%" identified by "abcde";

Because "192.168.%" has a higher priority in the permission table than "%". So when "cmy" try
to login in by password "12345" from host "192.168.1.1", it should match the second permission
entry, and will be rejected because of invalid password.
But in current implementation, Doris will continue to check password on first entry, than let it pass. So we should change it.

Permission checking logic

After a user login, it should has a unique identity which is got from permission table. For example,
when "cmy" from host "192.168.1.1" login, it's identity should be cmy@"192.168.%". And Doris
should use this identity to check other permission, not by using the user's real identity, which is
cmy@"192.168.1.1".

Black list
Functionally speaking, Doris only support adding WHITE LIST, which is to allow user to login from
those hosts in the white list. But is some cases, we do need a BLACK LIST function.
Fortunately, by changing the logic described above, we can simulate the effect of the BLACK LIST.

For example, First we add a user by:
create user cmy@'%' identified by '12345';

And now user 'cmy' can login from any hosts. and if we don't want 'cmy' to login from host A, we
can add a new user by:
create user cmy@'A' identified by 'other_passwd';

Because "A" has a higher priority in the permission table than "%". If 'cmy' try to login from A using password '12345', it will be rejected.

ISSUE #2205

[imay]

LGTM