Add GLOBAL indexes for partitioned tables

In other RDBMS, there is the concept of GLOBAL or LOCAL indexes on partitioned tables.

MariaDB currently has LOCAL indexes, which are local to each partition in a table.

A GLOBAL index on the other hand is a one-to-many relationship, allowing one index partition to map to many table partitions and avoid the scanning behavior when partition keys are not in a query.

Implementations on other databases:

http://www.dba-oracle.com/t_global_local_partitioned_index.htm
Good justification on why postgres (and MariaDB) should have it: https://www.postgresql.org/message-id/CALtqXTcurqy1PKXzP9XO%3DofLLA5wBSo77BnUnYVEZpmcA3V0ag%40mail.gmail.com