rrpc: Round-robin sector target with cost-based gc

This target allows an Open-Channel SSD to be exposed asas a block
device.

It implements a round-robin approach for sector allocation,
together with a greedy cost-based garbage collector.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>