util: xleep for Windows.

Windows does not have a sleep(seconds). But it does have
a Sleep(milliseconds). Sleep() in windows does not have a
return value. Since we are not using the return value for xsleep()
anywhere as of now, don't return any.

Introduced by commit 275eebb9 (utils: Introduce xsleep for RCU quiescent state)

CC: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
vswitchd: Add external_ids to Flow_Table table in database schema.

Every other table has an external_ids column, which can be useful to
controller writers for integration purposes, so add one to Flow_Table also.

Reported-by: Ariel Tubaltsev <atubaltsev@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>