Set status to pausing upon reception of pause signal

[Amogh-Bharadwaj]

This PR now utilises the pausing state appropriately

fastpausing.mp4

E2E test adapted. The trigger in it works like:

postgres=# select * from flow_status_updates where old_status != new_status and flow_job_name='pause_test_2';
 id | flow_job_name | old_status | new_status 
----+---------------+------------+------------
 11 | pause_test  | 4          | 5
 12 | pause_test  | 5          | 1
 14 | pause_test  | 1          | 3
 15 | pause_test  | 3          | 2
 16 | pause_test  | 2          | 1
(5 rows)