no longer valid entries in the roles_mappings HASH. This job will be delegated
to handle_grant_* functions

It will only hold _valid_ entries for as long as it held in memory. Any change
regarding acl_users or acl_roles in memory should update the structure
immediately. This is why the rebuild_roles_mappings no longer removes invalid
entries.

In order to keep things consistent with the existing code, the following jobs
are assigned to each function:

The role of rebuild_roles_mappings is to recreate the links between users and
roles. Any other updates are to be done in the functions:

handle_grant_*

This change prepares the code for the next step, which is cascading updates.

function

Also removed code that loads that column into memory.

No cascading changes take place during a user rename. This needs to be
addressed.

user creation.

No more memory leaks in the code.

Still problems with hashtable acl_roles
Need to create a copy of grant_roles, currently it uses the same buffer

The function calls delete_dynamic, after if calls a free function on every
array element.

TODO:
Memory allocated for the array is never freed

Changed ACL_USER.user from char * to LEX_STRING.
Refactored every section that made use of ACL_USER.user as a char*.

This was done so as to be able to quickly check the hash_key of the acl_user.

role_grants is no longer used as it will be added to the ACL_USER class

It was a leftover from attempting to use Hash_set

There is an issue with correct searching of keys in the HASH.

TODO: Free structures on flush

MDEV-5123 Remove duplicated conditions pushed both to join_tab->select_cond and join_tab->cache_select->cond for blocked joins.
    
BNL and BNLH joins pre-filter the records from a joined table via JOIN_TAB::cache_select->cond.
There is no need to re-evaluate the same conditions via JOIN_TAB::select_cond. This patch removes
the duplicated conditions from the top-level conjuncts of each pushed condition.

The added "Using where" in few EXPLAINs is due to taking into account tab->cache_select->cond
in addition to tab->select_cond in JOIN::save_explain_data_intern.

depends on uninitialised value(s) in JOIN::save_explain_data_intern"
- Make find_best()  /* the old join optimizer code */ also use table
  condition selectivity.