Switch is_user_member_of_blog to only fetch a specific user meta key for capability rather than all meta keys for the user

[rinatkhaziev]

https://core.trac.wordpress.org/changeset/33771 introduced performance improvements to is_user_member_of_blog to combat the slowness of get_blogs_of_user

It does so by fetching all meta keys ($meta_key = ''). This makes it challenging to use "get_{$meta_type}_metadata" filter, as there's no good way to tell which part we want to short-circuit or apply special handling to.

The proposed approach is to change the logic do check for the specific blog's capability check name.

Trac ticket: https://core.trac.wordpress.org/ticket/63989

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[jonnynews]

Requested some changes @rinatkhaziev

[rinatkhaziev]

@jonnynews thank you, makes perfect sense. I've applied the suggestions.

[spacedmonkey]

Committed