BUG/MINOR: quic: do not decrement jobs for backend conns

BUG/MINOR: quic: do not decrement jobs for backend conns

jobs is a global counter which serves to account activity through the
whole process. Soft-stop procedure will wait until this counter is
resetted to the nul value.

jobs is not used for backend connections. Thus, it is not incremented
when a QUIC backend connection is instantiated as expected. However,
decrement is performed on all sides during quic_conn_release(). This
causes the counter wrapping.

Fix this by decrementing jobs only for frontend connections. Without
this patch, soft stop procedure will hang indefinitely if QUIC backend
connections were in use.