lock_manager: add better tracing for slow lock acquisition
lock_manager: add better tracing for slow lock acquisition

This can help diagnose performance issues due to row lock contention.
I was just helping a user who was accidentally inserting a lot of
duplicate primary keys, which resulted in high contention on those
row locks.

This was somewhat evident from the 'prepare_run_time_us' histogram
metric, but would have been more obvious with some request counters
showing the amount of time spent blocked on locks.

No new test since this is just trace instrumentation.

Change-Id: I133f496bfe8e85128471fc151001a7dab5da4eff
Reviewed-on: http://gerrit.cloudera.org:8080/2868
Reviewed-by: David Ribeiro Alves <33ea948168c114d220e0372a903be6ee60f6396e@cloudera.com>
Tested-by: Kudu Jenkins

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