Use get_user_by() for bp_core_get_userid_from_nicename()

[r-a-y]

bp_core_get_userid_from_nicename() (or bp_core_get_userid()) is run on every member profile page and is currently uncached.

Attached patch:

• switches the direct DB query with get_user_by(), which caches the query when an object cache is enabled
• returns NULL so it emulates the previous DB query on failure

[boonebgorges]

Thumbs up from me. Removing SQL queries FTW :)

[DJPaul]

Looks good

[r-a-y]

In 8532:

Use get_user_by() for bp_core_get_userid_from_nicename() and bp_core_get_userid()

Previously, these functions used direct DB queries to grab the user ID when
a user_nicename or user_login was passed.

This commit switches these direct DB queries to use get_user_by(), which
caches the query when a persistent object cache is enabled.

Fixes #5696.