Construct Windows locks using InitializeCriticalSectionAndSpinCount

Previously we were using InitializeCriticalSection, which creates a
lock that blocks immediately on contention and waits to be
rescheduled.  This is inefficient; it's better to wait for a little
while before telling the US to reschedule us, in case the lock becomes
available again really soon (since most locks mostly do).

Good pthreads implementations do this automatically.  On Windows,
though, we need to call this magic function, and we need to pick the
spin count ourselves.

event_iocp.c

@@ -193,7 +193,7 @@ event_iocp_port_launch(void)
 		++port->n_live_threads;
 	}
 
-	InitializeCriticalSection(&port->lock);
+	InitializeCriticalSectionAndSpinCount(&port->lock, 1000);
 
 	return port;
 err:

evthread_win32.c

@@ -38,13 +38,18 @@ struct event_base;
 
 #include "mm-internal.h"
 
+#define SPIN_COUNT 2000
+
 static void *
 evthread_win32_lock_create(unsigned locktype)
 {
 	CRITICAL_SECTION *lock = mm_malloc(sizeof(CRITICAL_SECTION));
 	if (!lock)
 		return NULL;
-	InitializeCriticalSection(lock);
+	if (InitializeCriticalSectionAndSpinCount(lock, SPIN_COUNT) == 0) {
+		mm_free(lock);
+		return NULL;
+	}
 	return lock;
 }