Add hook to optionally enable threaded sheet stream parsing.

Threaded version still not working as the fast parser deadlocks during
threaded parsing.

Change-Id: I3d402a22a394d7d0d7edf96590ae039506928fde
No indent on namespaces.

Change-Id: I2639a51c2faa19373a335c28b160862435cd6dbe
A bit of cleanup.

Change-Id: I576a184ebc8422d760800c670f8d9070f66e9e78
Allow worker threads to use their own FastParser instances.

To prevent deadlock during threaded sheet stream parsing.  It now
deadlocks at a different place.

Change-Id: I0ba0f2c9a257e71b0a340ab14e369b06d5fd8829