expires: refactoring judgment about whether a key is expired

Calling lookupKey*() many times to search a key in one command
may get different result.

That's because lookupKey*() calls expireIfNeeded(), and delete
the key when reach the expire time. So we can get an robj before
the expire time, but a NULL after the expire time.

The worst is that may lead to Redis crash, for example
`RPOPLPUSH foo foo` the first time we get a list form `foo` and
hold the pointer, but when we get `foo` again it's expired and
deleted. Now we hold a freed memory, when execute rpoplpushHandlePush()
redis crash.

To fix it, we can refactor the judgment about whether a key is expired,
using the same basetime `server.cmd_start_mstime` instead of calling
mstime() everytime.

src/server.c

@@ -3596,6 +3596,7 @@ int processCommand(client *c) {
         queueMultiCommand(c);
         addReply(c,shared.queued);
     } else {
+        server.cmd_start_mstime = mstime();
         call(c,CMD_CALL_FULL);
         c->woff = server.master_repl_offset;
         if (listLength(server.ready_keys))