PlainTableFactory::PlainTableFactory() to have huge TLB turned off by default

Summary: PlainTableFactory::PlainTableFactory() now has Huge TLB page feature turned on by default. Although it is not a public API (which we always turn the feature off now), our unit tests, like db_test sometimes uses it directly, which causes wrong coverage of codes. This patch fix it to allow unit tests to run with the correct setting

Test Plan: Run db_test and make sure this feature is not on any more.

Reviewers: igor, haobo

Reviewed By: igor

CC: yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18483