[FIX] mail.thread: avoid wrongly matching model,id from a foreign OpenERP system

When emails transit from an OpenERP system to another
there is a chance of wrongly considering the foreign
model,id pair as local, possibly attaching a reply
to a wrong parent message.
This naive heuristic ignore the foreign identifiers
unless the hostname of the message-id mentioned in
the In-Reply-To header matches the local one.
This may fail after a change of local hostname but
is still better than the alternative.
In OpenERP v8 we should stop including the model,res_id
in outgoing Message-Id headers and only rely on the
stored, opaque, unique Message-Ids, preventing
any future mismatch.