txn: record long running transaction and session information for diagnosis

[cfzjywxk]

Enhancement

The safepoint calculation is the first step for garbage collection in TiDB, data older than the safepoint could be safely reclaimed. The safepoint must be less than the global minimal start_ts among all the tidb sessions, so a session with long-running transactions may block the advancement of safepoint. For example, if the user starts a transaction but never commits, the safepoint could not be pushed and data reclamation is blocked. To make the diagnosis of such issues easier, consider the following improvements in TiDB:

1. Record the long-running transaction and session information in the expensive logs. If the safepoint is blocked by some ongoing sessions, the start_ts could be used to locate the blocking session.
2. Record ongoing transaction duration in the metrics with a certain flushing interval, so the long-running transitions could be viewed directly on the grafana panels.