(bug 1495) Enable on-wiki message language fallbacks

The core function behind wfMessage() (MessageCache->get()) did not
apply the language fallback chain to on-wiki messages.

This patch has changed the behavior to iterate over all possible
on-wiki fallbacks (starting with the user's language) before
using the built-in language cache (CDB files). Previously we only looked
for the existence of an on-wiki message in the users's language.

Performance wise, using the 'ab' language ('ru', 'en' fallbacks)
MessageCache::get (Averaged over runs and calls)
New Code: ~8.5% TET (110us/call)
Old Code: ~6.5% TET ( 90us/call)

TET: Total Execution Time

Change-Id: Iaaf6ccebd8c40c9602748c58c3a5c73c29e7aa4d
Merge "(bug 1495) Enable on-wiki message language fallbacks"