fix(istgt config): add TCPUserTimeout setting in istgt configuration (#…

…354) (#366)

This commit adds new setting called TCPUserTimeout in istgt configuration.
Default value of TCPUserTimeout is configured to 120 seconds which means
maximum amount of time in seconds that transmitted data may remain unacknowledged,
or buffered data may remain untransmitted(due to zero window size) before
TCP will forcibly close the corresponding connection and return error event
to application.

**Why is this change needed**:
Current implementation of cStor-istgt allows only one initiator
to consume a LUN(volume) at a time. When initiator machine is
abruptly powered-off connection is no more active between client & target.
To know about client connection status istgt configures TCP_USER_TIMEOUT
to 120 seconds during login time.

**NOTE**:
TCP_USER_TIMEOUT will helpful only if there is packet flow from istgt.
Here, if connection remains idle for 20 seconds then istgt will send
Nop-In request to client(to check availability). If client is available
things will work smooth else if packet is unacknoweldged for 120 secongs
then connection will be dropped.

Signed-off-by: mittachaitu <sai.chaithanya@mayadata.io>

pkg/controllers/volume-mgmt/volume/volume.go

@@ -50,6 +50,7 @@ var (
   MediaDirectory "/mnt"
   Timeout 60
   NopInInterval 20
+  TCPUserTimeout 120
   MaxR2T 16
   DiscoveryAuthMethod None
   DiscoveryAuthGroup None