MultiGet for DBWithTTL

Summary: This is a feature request from rocksdb's user. I didn't even realize we don't support multigets on TTL DB :)

Test Plan: added a unit test

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30561
Fix a SIGSEGV in BackgroundFlush

Summary:
This one wasn't easy to find :)

What happens is we go through all cfds on flush_queue_ and find no cfds to flush, *but* the cfd is set to the last CF we looped through and following code assumes we want it flushed.

BTW @sdong do you think we should also make BackgroundFlush() only check a single cfd for flushing instead of doing this `while (!flush_queue_.empty())`?

Test Plan: regression test no longer fails

Reviewers: sdong, rven, yhchiang

Reviewed By: yhchiang

Subscribers: sdong, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30591