Implement add_meta() functions and other WP metadata goodies

[boonebgorges]

In migrating to WP's metadata functions, we now have the ability to use some of the good stuff that those functions have available, such as:

• the $single param of get_metadata()
• the $delete_all param of delete_metadata()
• the add_metadata() function

Should do a review and see how these can be consistently added to each of our component metadata libraries.

[boonebgorges]

In 7872:

Introduce bp_activity_add_meta()

See #5400

[boonebgorges]

In 7875:

Introduce groups_add_groupmeta(). See #5400

[boonebgorges]

In 7877:

Introduce bp_blogs_add_blogmeta()

See #5400

[boonebgorges]

In 7878:

Introduce bp_xprofile_add_meta().

See #5400

[boonebgorges]

In 7879:

Add support for $single param in _get_meta() functions

Unlike get_metadata(), we are defaulting to $single = true, for backward
compatibility.

See #5400

[boonebgorges]

In 7883:

Add delete_all support for all _delete_meta() functions

The delete_all parameter allows you to delete all meta items matching the
specified meta_key, regardless of the associated object.

Because our meta delete functions also support the deletion of all metadata
associated with an object (by leaving out the meta_key param), a decision had
to be made regarding the behavior when _delete_meta() is called with delete_all

true and meta_key = false. The most logical (and least destructive) strategy

was deemed to be: force delete_all to false when meta_key is also false. This
ensures that you don't accidentally wipe out all metadata for a component.

See #5400

[boonebgorges]

In 7885:

Add support for prev_value param to _update_meta() functions

This parameter, inherited from update_metadata(), allows you to limit your
updates only to rows where the meta_value matches your specified prev_value.

See #5400

[boonebgorges]

I think I've gotten all the good stuff out of the WP functions. Aside from the quirks in our functions (see #5399) we have complete feature parity with WP's metadata functions. Marking fixed.