[MERGE] Registry loading speedup by thu

All calls to ir_model_fields to fetch manual (aka custom or x_) fields are done
in a single call (prior to instanciate all models).

Checks for empty module descriptions are not done unless we are in init or
update mode. (The behavior was the opposite, which was probably a mistake).

Some calls to ir_translation, passing en_US because there was no lang in the
context, are not done anymore.

bzr revid: al@openerp.com-20121216030722-thxk3ddff8lkw05y