Filter current pid when listing locks

Don't show our own query in list of locks.

Old:

```
$ heroku pg:locks -a an_app_name
 pid  | relname | transactionid | granted |   query_snippet   |   age
------+---------+---------------+---------+-------------------+----------
 5327 |         |               | t       |                  +| 00:00:00
      |         |               |         |      SELECT      +|
      |         |               |         |        pg_stat_ac |
```

new:

```
$ heroku pg:locks -a an_app_name
 pid | relname | transactionid | granted | query_snippet | age
-----+---------+---------------+---------+---------------+-----
(0 rows)
```