Allow worker threads to use their own FastParser instances.

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

Conflicts:
	oox/source/core/xmlfilterbase.cxx
	sc/source/filter/oox/workbookfragment.cxx

Change-Id: I0ba0f2c9a257e71b0a340ab14e369b06d5fd8829
sc: threaded parsing of the core data inside large XLSX files

Enabled in experimental mode only or via SC_IMPORT_THREADS=<N> this
allows significant parallelisation of sheet reading. I also implement
a simple thread pool to manage that.

Conflicts:
	sc/source/filter/oox/workbookfragment.cxx

Change-Id: I66c72211f2699490230e993a374c26b1892eac12
sc: no such thing as std::vector::find

Change-Id: I11bb688a8e0affda9ed78a61d9aba3d244914f8c