Don't run background jobs (flush, compactions) when bg_error_ is set

Summary:
If bg_error_ is set, that means that we mark DB read only. However, current behavior still continues the flushes and compactions, even though bg_error_ is set.

On the other hand, if bg_error_ is set, we will return Status::OK() from CompactRange(), although the compaction didn't actually succeed.

This is clearly not desired behavior. I found this when I was debugging t5132159, although I'm pretty sure these aren't related.

Also, when we're shutting down, it's dangerous to exit RunManualCompaction(), since that will destruct ManualCompaction object. Background compaction job might still hold a reference to manual_compaction_ and this will lead to undefined behavior. I changed the behavior so that we only exit RunManualCompaction when manual compaction job is marked done.

Test Plan: make check

Reviewers: sdong, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23223