CRM-16138 - CRM_Extensions_System - Include domain_id in cache paths

CRM-16138 - CRM_Extensions_System - Include domain_id in cache paths

The extension cache (returned by CRM_Extension_System::getCache) is already
keyed off extensionDir, extensionUrl, civicrm_root, and several others.
This patch adds domain_id to the list.

The patch is derived from agileware's patch from 4.4. Notably, the original
patch only influenced mapper (not the other caches).

I don't really think this patch should be necessary. Firstly, there are
already many variables factored into the cache-key for 4.6+ (CRM-15788) --
one (or more) should get tripped in "single-database, multiple-codebase"
architecture.  Secondly, the extension list is not innately domain-based.

However, there are a dizzying range of variations on
multi-site/multi-domain, and I may not have a full appreciation for the one
that led to agileware's patch.  And (theoertically) there's no big downside
to including domain_id as part of the cache.