dsched, dsched_fq - Major cleanup

dsched, dsched_fq - Major cleanup

SHORT version: major cleanup and rename to useful names

LONG version:

dsched:
* Unify dsched_ops with dsched_policy, remove fallout

* Rename dsched_{create,destroy} -> dsched_disk_{create,destroy}_callback

* Kill .head in dsched_policy

* Kill dead code

dsched_fq:
* Rename fqp -> thread_io/tdio
* Rename fqmp -> thread_ctx/tdctx
* Rename dpriv -> disk_ctx/diskctx

* Several related renames of functions (alloc/ref/unref).

* Remove dead code

* rename tdctx->s_* -> tdctx->interval_*

* comment struct members

* ... and some more changes I probably forgot

Huge-Thanks-To: Aggelos Economopoulos