Improve cache invalidation for xprofile groups

XProfile groups are cached in two ways: individually and as a collection. This
changeset improves group cache invalidation by ensuring that both of these
caches are wiped when child fields are added or edited.

See #1332

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7791 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Consult cache first when checking BP_XProfile_ProfileData::exists()

See #1332

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7792 cdf35c40-ae34-48e0-9cc9-0c9da1808c22