Like other explainable commands, aggregate can now be
explained using the explain command, e.g.
db.runCommand({explain: {aggregate: ...}, verbosity:
"executionStats"}). The existing explain:true flag
corresponds to "queryPlanner" mode and is still supported.
However, explain:true cannot be specified when explaining
aggregate via the explain command.
Additional execution information is provided only in the
$cursor section of the aggregation explain output. Having
aggregation stages themselves track and report execution
info is further work.
