Don't compile ldb tool into static library

Summary:
This is first step of my effort to reduce size of librocksdb.a for use in mobile.

ldb object files are huge and are ment to be used as a command line tool. I moved them to `tools/` directory and include them only when compiling `ldb`

This diff reduced librocksdb.a from 42MB to 39MB on my mac (not stripped).

Test Plan: ran ldb

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17823