bdevperf: Rename io_target to bdevperf_job

Begin shifting to a model of "jobs", each defined as a single-threaded
I/O pattern.

Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Change-Id: If6c0cfc1a59d3acdec1e1e6ad65a16f810582aa8
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1382


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>