TestByteBufferInputStream: Reduce FLUSH_SOFT load
MappedByteBufferInputStream: Default CacheMode is FLUSH_PRE_HARD now (was FLUSH_PRE_SOFT)

FLUSH_PRE_SOFT cannot be handled by some platforms, e.g. Windows 32bit.

FLUSH_PRE_HARD is the most reliable caching mode
and it will fallback to FLUSH_PRE_SOFT if no method for 'cleaner' exists.

Further, FLUSH_PRE_HARD turns our to be the fastest mode as well.