[core] eavesdrop: reduce lock contention, fix error condition + enable eavesdrop stereo

src/include/switch_types.h

@@ -353,7 +353,8 @@ typedef enum {
 	ED_BRIDGE_READ = (1 << 4),
 	ED_BRIDGE_WRITE = (1 << 5),
 	ED_TAP_READ = (1 << 6),
-	ED_TAP_WRITE = (1 << 7)
+	ED_TAP_WRITE = (1 << 7),
+	ED_STEREO = (1 << 8)
 } switch_eavesdrop_flag_enum_t;
 typedef uint32_t switch_eavesdrop_flag_t;