DatabaseCampaign: MySQL / concurrency fixes

According to
<http://dev.mysql.com/doc/refman/5.5/en/c-api-threaded-clients.html>,
a MySQL connection handle must not be used concurrently with an open
result set and mysql_use_result() in one thread
(DatabaseCampaign::run()), and mysql_query() in another
(DatabaseCampaign::collect_result_thread()).  This indeed leads to
crashes when bounding the outgoing job queue (SERVER_OUT_QUEUE_SIZE),
and maybe even more insidous effects in other cases.  The solution is
to create separate connections for both threads.

Additionally, call mysql_library_init() before spawning any threads.

Change-Id: I2981f2fdc67c9a2cbe8781f1a21654418f621aeb

src/core/util/Database.cc

@@ -229,6 +229,9 @@ void Database::cmdline_setup() {
 								  "-h/--hostname \tMYSQL Hostname (default: taken from ~/.my.cnf)");
 	USERNAME	  = cmd.addOption("u", "username", Arg::Required,
 								  "-u/--username \tMYSQL Username (default: taken from ~/.my.cnf, or your current user)\n");
+
+	// should be called before any threads are spawned
+	mysql_library_init(0, NULL, NULL);
 }
 
 Database * Database::cmdline_connect() {