Fix queue manipulation in WriteThread::BeginWriteStall() (#6322)

Fix queue manipulation in WriteThread::BeginWriteStall() (#6322)

Summary:
When there is a write stall, the active write group leader calls ```BeginWriteStall()``` to walk the queue of writers and remove any with the ```no_slowdown``` option set. There was a bug in the code which updated the back pointer but not the forward pointer (```link_newer```), corrupting the list and causing some threads to wait forever. This PR fixes it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6322

Test Plan: Add a unit test in db_write_test

Differential Revision: D19538313

Pulled By: anand1976

fbshipit-source-id: 6fbed819e594913f435886606f5d36f74f235c3a