Socket-based kill idle transaction feature re-implementation

https://github.com/percona/percona-server/pull/1164

Current kill idle transaction feature implementation works at a low InnoDB transaction list level, periodically scanning it and killing the old ones.

Lixun Peng proposed re-implementing this by setting a different socket read timeout value instead (bug 907719). Such implementation has several advantages:
- much simpler code, getting rid of crashing bugs in the current one (bug 1166744, bug 1179136)
- works with any storage engine, not only InnoDB.

On user level, introduce a new server variable kill_idle_transaction. Make the old innodb_kill_idle_transaction variable its deprecated alias, to be removed in 8.0.