add support for nested BlockBasedTableOptions in config string

Summary:
Add support to allow nested config for block-based table factory. The format looks like this:

"write_buffer_size=1024;block_based_table_factory={block_size=4k};max_write_buffer_num=2"

Test Plan: unit test

Reviewers: yhchiang, rven, igor, ljin, jonahcohen

Reviewed By: jonahcohen

Subscribers: jonahcohen, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29223