PrimProc thread leak if ExeMgr dies

If the ExeMgr is processing queries and it dies then PrimProc can be left with a bunch of orphaned BPP threads.

How to reproduce this:

Step 1:

sudo cat /proc/`pidof PrimProc`/status | grep Threads

Threads:	117

Step 2: Run a bunch of concurrent queries
Step 3: Whilst the queries are running:

sudo kill -9 `pidof ExeMgr`

Step 4: Wait until CPU usage for PrimProc settles down to 0 and then re-run the thread count from step 1. If the bug exists then this will be higher (probably around 137). If it is fixed it will be similar to the first figure.

Every time ExeMgr is killed the leak count will increase which also means RAM is being leaked.