Expose 'index.compound_on_flush' via engine settings

Lucene 4.4 shipped with a fundamental change in how the decision
on when to write compound files is made. During segment flush the
compound files are written by default which solely relies on a flag
in the IndexWriterConfig. The merge policy has been factored out to
only make decisions on merges and not on IW flushes. The default now
is always writing CFS on flush to reduce resource usage like open files
etc. if segments are flushed regularly. While providing a senseable
default certain users / usecases might need to change this setting if
re-packing flushed segments into CFS is not desired.

Closes #3461