Messages: Introduces meta table and related cache functions.

This commit alters the messages DB schema to add a meta table, which will
allow plugin developers to record information about a message.

The new message meta functions in bp-messages-functions.php mirrors those
in other BP components (activity, groups, etc.).

The DB version is also bumped to 9181.

See #3083.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9182 cdf35c40-ae34-48e0-9cc9-0c9da1808c22