Fix statement_timeout on Win32 so that it properly treats micro-seconds

Fix statement_timeout on Win32 so that it properly treats micro-seconds
as micro-seconds, rather than as 100 microseconds, as it does now.  This
actually fixes all setitimer calls on Win32, but statement_timeout is
the most visible fix.

Backpatch to 8.1.X.  8.0 works as documented.