session vcl: refactor builtin tx event for main tx

Rename unused SESSION_IO_EVT_BUILTIN_TX to SESSION_IO_EVT_TX_MAIN and
leverage it for non-connected udp tx.

Non-connected udp sessions are listeners and are therefore allocated on
main thread. Consequently, whenever session queue node is not polling
main, tx events generated by external applications might be missed or
processed with some delay. To solve this, request that apps use
SESSION_IO_EVT_TX_MAIN tx events as opposed to SESSION_IO_EVT_TX and
send that to first worker as opposed to main.

Type: fix

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I5df5ac3dc80c0f192b2eefb1d465e9deefe8786b
diff --git a/src/vnet/session/session_debug.c b/src/vnet/session/session_debug.c
index dba4ef3..39d1752 100644
--- a/src/vnet/session/session_debug.c
+++ b/src/vnet/session/session_debug.c
@@ -280,7 +280,7 @@
     case SESSION_IO_EVT_RX:
     case SESSION_IO_EVT_TX:
     case SESSION_IO_EVT_BUILTIN_RX:
-    case SESSION_IO_EVT_BUILTIN_TX:
+    case SESSION_IO_EVT_TX_MAIN:
     case SESSION_IO_EVT_TX_FLUSH:
       if (e->session_index == f->shr->master_session_index)
 	return 1;