Add trx pointer to struct mtr_t

commit 25295d003cb0c17aa8fb756523923c77250b3294
Author: Steaphan Greene <steaphan@fb.com>
Date: Sun Mar 23 22:13:27 2014 -0700

Add trx pointer to struct mtr_t

Summary:
Feature: Basic Code Changes

This adds a pointer to the trx to each mtr.
This allows the trx to be accessed in parts of the code
where it was otherwise not available.