Profile the performance of `/api/spaces/space` with a large number of spaces

[legrego]

Kibana performs the following whenever we need to get the full list of authorized spaces:

1. Query ES for all known spaces
2. Perform a _has_privileges check for the current user for all known spaces
3. Return a filtered list of spaces that the user is authorized for

This works well enough for a reasonable amount of spaces, but has potential issues when scaling out to a large number of spaces. We should see what performance bottlenecks exist, and if there is anything we can do to optimize this flow.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)