WL#6808: InnoDB: Deprecate and remove innodb_mirrored_log_groups

Deprecate innodb_mirrored_log_groups.

Background: the allowed range of the parameter innodb_mirrored_log_groups is
1..10, but there has always been (at least since 3.23.53) a check in InnoDB
startup that refuses any other value than 1.

This is not meaningful and we should deprecate this variable in GA versions and
remove it in a later release.

Per Server PT decison and a post-meeting discussion with Support:     

Throw a deprecation message in 5.6 and remove completely in 5.7

 5.6 :
======

When Innodb_mirrored_log_groups !=1, abort with the following error message 

"Innodb_mirrored_log_groups is an unimplemented feature and the variable will be
completely removed in a future version. Using values other than 1 is not supported."


When Innodb_mirrored_log_groups == 1, WARN and continue with the following error
message

"Innodb_mirrored_log_groups is an unimplemented feature and the variable will be
completely removed in a future version."

When Innodb_mirrored_log_groups is NOT passed, do nothing.

5.7
===
Remove all mention and give usual unknown variable errors.