Fix logging

Summary: We were clearing some of the stats before logging them

Reviewed By: @jano

Differential Revision: D1637551
Remove tc-space reporting of astubs

Summary: Astubs hasn't existed for almost 6 months now, so it's about time to
retire this stat.  We can always synthesize it in ODS if needed.

Reviewed By: @ottoni

Differential Revision: D1631327
Use operator= instead of clear to reset MM.m_instances

Summary: clear() doesn't guarantee to release the underlying storage of the
map, whereas operator= with a new empty map should.
Only do this if m_trackingInstances, to avoid construction unless needed.

Reviewed By: @shai-d

Differential Revision: D1631356
Use operator= instead of clear to reset xmlErrorVec

Summary: clear() doesn't free storage, so this could grow steadily over time.

Reviewed By: @edwinsmith

Differential Revision: D1632303