Make the options in table_builder/block_builder less misleading

Summary:
By original design, the regular `block options` and index `block options` in table_builder is mutable. We can use ChangeOptions to change the options directly.

However, with my last change, `BlockBuilder` no longer hold the reference to the index_block_options -- as a result, any changes made after the creation of index block builder will be of no effect.

But still the code is very error-prone and developers can easily fall into the trap without aware of it. To avoid this problem from happening in the future, I deleted the `ChangeOptions` and the `index_block_options`, as well as many other changes to make it less misleading.

Test Plan:
make
make check
make release

Reviewers: dhruba, haobo

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13707