Increases events.shared-stacksize to 2K

ESP8266 driver started to use global event queue to handle some AT
parsing and call Socket::sigio() events. It turned out that when
running DNS tests, or Pelion Client, the stack space (1kB) is not
enough for the purpose.

Therefore we propose that this is raised to 2kB, as the assumption
is that event loop should allow as equivalently complex code than
any other threads.

In Mbed OS, the default Thread stack size is 4kB, but we assume 2kB
to be enough for non-blocking event purposes.

events/mbed_lib.json

@@ -4,7 +4,7 @@
         "present": 1,
         "shared-stacksize": {
             "help": "Stack size (bytes) for shared event queue thread",
-            "value": 1024
+            "value": 2048
         },
         "shared-eventsize": {
             "help": "Event buffer size (bytes) for shared event queue",