Bug 1392546 - Travis/Vagrant: Set innodb_flush_log_at_trx_commit=0

By default MySQL#s InnoDB flushes to disk after every transaction to
reduce the chance of data loss in the case of a crash. During testing
and development this is not necessary and only serves to limit I/O
throughput. By switching to mode `0` (flushing every second), test
runtimes are reduced by 20-30%.

See:
https://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_flush_log_at_trx_commit

vagrant/mysql.cnf

@@ -19,3 +19,6 @@ sql_mode="NO_ENGINE_SUBSTITUTION,STRICT_ALL_TABLES"
 # Unhelpfully MySQL uses a different (undocumented) variable name if set via config file:
 # https://bugs.mysql.com/bug.php?id=70008
 transaction-isolation=READ-COMMITTED
+
+# Vagrant/Travis only: Speed up I/O by reducing data-loss protection.
+innodb_flush_log_at_trx_commit="0"