Cache path for Extension mapper should include Domain reference.

This is primarily an issue for multiple sites accessing the same CiviCRM database from multiple codebases.

We found on a series of sites using the same database from different installed codebases, the mapped extension path would become wrong for all but one site root - which would for example cause access errors if not all of the sites were under the same user account.

This is due to the extension path being mapped as "mapper/moduleFiles" for all domains.

Observing git, this appears to still be the case in 4.5 and 4.6

Attached patch (for 4.4 but should apply with fuzz for 4.5 and 4.6) inserts the domain key into the cache path for the extensions, solving the issue where we've seen it.