MDL-43882 performance: New metric, time spent by the database

This patch adds a new performance metric to the performance
info shown by MDL_PERF* vars, the time spent by the database,
it was one of the wonderful @poltawski ideas. To be more specific
the value displayed is the sum of the time elapsed between query_start()
and query_end().
MDL-43882 dml: New test for perf_get_xxxx() methods