vcl: use WT_EXECUTEINTIMERTHREAD for Timer Queue

Currently the timer callback is running on a separate thread, but that
is unnecessary since it just does a PostMessage().

Every thread stack stack mysteriously eats 10M of VM apparently despite
dumpbin -headers soffice.bin:
"989680 size of stack reserve, 1000 size of stack commit"

Change-Id: I6eac3f6fa45939cd5dbb3dfbec61f9a504bdaa8b