Improve Options sanitization and add MmapReadRequired() to TableFactory

Summary:
Currently, PlainTable must use mmap_reads.  When PlainTable is used but
allow_mmap_reads is not set, rocksdb will fail in flush.

This diff improve Options sanitization and add MmapReadRequired() to
TableFactory.

Test Plan:
export ROCKSDB_TESTS=PlainTableOptionsSanitizeTest
make db_test -j32
./db_test

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: you, leveldb

Differential Revision: https://reviews.facebook.net/D21939