events.shared-eventsize: increased from 256B to 768B

Original value was too small once both ESP8266 driver and
asynchronous DNS started to use shared event queue. An assumption is
made that once shared event queue is taken into use there are going to
be multiple users instead of one, for which the original value would
have been sufficient.

events/mbed_lib.json

@@ -8,7 +8,7 @@
         },
         "shared-eventsize": {
             "help": "Event buffer size (bytes) for shared event queue",
-            "value": 256
+            "value": 768
         },
         "shared-dispatch-from-application": {
             "help": "No thread created for shared event queue - application will call dispatch from another thread (eg dispatch_forever at end of main)",