New defaults for eq_range_index_dive_limit in 10.4

See https://lists.launchpad.net/maria-developers/msg11460.html :

So, MDEV-16934 introduced eq_range_index_dives_limit into 10.2.8 and 10.3.0.

The default was set to 0 (which means no limit) in order to not introduce
optimizer behavior changes into stable versions.

The question is: should 10.4 also have 0 by default or we can set it to some
finite limit? MySQL's default value is 10.

After the decision was: It should be set to 10. This is what this task is about.

Saw MYSQL 8.0 , the default there is 200
https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_eq_range_index_dive_limit
psergey igor for your information