Remove the terrible hack in for flush_block_policy_factory

Summary:
Previous code is too convoluted and I must be drunk for letting
such code to be written without a second thought.

Thanks to the discussion with @sdong, I added the `Options` when
generating the flusher, thus avoiding the tricks.

Just FYI: I resisted to add Options in flush_block_policy.h since I
wanted to avoid cyclic dependencies: FlushBlockPolicy dpends on Options
and Options also depends FlushBlockPolicy... While I appreciate my
effort to prevent it, the old design turns out creating more troubles than
it tried to avoid.

Test Plan: ran ./table_test

Reviewers: sdong

Reviewed By: sdong

CC: sdong, leveldb

Differential Revision: https://reviews.facebook.net/D16503