Limit random MySQL server-id to 2^31-1

[enisoc]

FYI @rnavarro

Users have seen the semi-sync plugin crash on the master, as mentioned in #2279.

From the MySQL error log:

2016-11-10T01:59:24.126332Z 1188 [Note] Start binlog_dump to master_thread_id(1188) slave_server(2904761766), pos(, 4)
2016-11-10T01:59:24.126371Z 1188 [Note] Start semi-sync binlog_dump to slave (server_id: -1390205530), pos(, 4)
*** buffer overflow detected ***: /usr/sbin/mysqld terminated

I noticed that the semi-sync log line interprets the server_id as signed 32-bit, whereas the log line from core mysqld interprets it as unsigned (or perhaps signed 64-bit):

>>> struct.unpack('i', struct.pack('I', 2904761766))
(-1390205530,)

It could just be a printing error, but I wouldn't be surprised if this is actually causing the buffer overflow.

When choosing random server_ids, we had assumed we could go up to 2^32-1 because that's what the MySQL docs say. However it appears the semi-sync plugin does not agree, so we should be more conservative and limit ourselves to 2^31-1.