Don't roll empty logs

Summary:
With multiple column families, especially when manual Flush is executed, we might roll the log file, although the current log file is empty (no data has been written to the log).

After the diff, we won't create new log file if current is empty.

Next, I will write an algorithm that will flush column families that reference old log files (i.e., that weren't flushed in a while)

Test Plan: Added an unit test. Confirmed that unit test failes in master

Reviewers: dhruba, haobo, ljin, sdong

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17631
Set log_empty to false even when options.sync is off [fix tests]
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
Fix sst_dump and reduce_levels_test compile errors