Terminate idle transactions safely in server layer by setting up socket timeout
parameter. Percona provides another patch to resolve similar problem, but it
calls server layer's callback in InnoDB plugin to close THD, which crashes in
some testcases. See https://bugs.launchpad.net/percona-server/+bug/901060 for
more detailed information.
1. export parameter trx_idle_timeout to handle all kinds of transactions, the priority is highest
2. export parameter trx_readonly_idle_timeout to handle read-only transactions
3. export parameter trx_changes_idle_timeout to handle read-write transactions
