[PATCH] parallel replication status variables

from pivanof on mailing list in reference to parallel replication:

> some status variables which could be plotted over time and show (or at
> least hint on) whether this is significant bottleneck for performance
> or not.

> This could be something like total time (in both wall time and
> accumulated CPU time) spent executing transactions in parallel, time
> spent rolling back transactions due to this lock conflict, time spent
> rolling back transactions because of other reasons (e.g. due to STOP
> SLAVE or reconnect after master crash), maybe also time spent waiting
> in one parallel thread while transaction is executing in another
> thread, etc.