Improvements to object caching in bp-friends

[r-a-y]

The friends component uses object caching in some places, but it's incomplete and in some cases appears to be incorrect.

This is a general ticket for commits that address this issue for the 2.0 cycle.

Related: #5407, #5434.

[r-a-y]

In 7999:

Don't query friendship status in BP_User_Query for logged-out users.

The bp_friends_filter_user_query_populate_extras() function is used to
add the confirmed friendship status against the logged-in user to the
existing user query.

However, we don't need to run this query if a user isn't logged in.

See #5435.

[r-a-y]

In 8103:

Friends: Add cache file to component loader.

This file was never included during the BP component API refactor
from r3917.

See #5435.

[johnjamesjacoby]

Nice catch.

[r-a-y]

In 8105:

Cache the friendship request user IDs for a given user.

This commit:

• Caches calls to BP_Friends_Friendship::get_friendship_request_user_ids()
• Invalidates the friend request cache when a friendship is saved or removed
• Adds unit tests

See #5435, #5414

[r-a-y]

Closing for 2.0. We can revisit further optimizations in subsequent releases.