tgtd: send/recv iSCSI PDUs in worker threads

Current tgtd sends and receives iSCSI PDUs in its main event
loop. This design can cause bottleneck when many iSCSI clients connect
to single tgtd process. For example, we need multiple tgtd processes
for utilizing fast network like 10 GbE because typical single
processor core isn't fast enough for processing bunch of requests.

This patch lets tgtd send/receive iSCSI PDUs and check digests in its
worker threads. After applying this patch, the bottleneck in the main
event loop is removed and the performance is improved.

The improvement can be seen even if tgtd and iSCSI initiator are
running on a single host. Below is a snippet of fio result on my
linux box. The workload is 128MB random RW. Backingstore is sheepdog.

Original tgtd:
  read : io=65008KB, bw=6259.9KB/s, iops=1564 , runt= 10385msec
  write: io=66064KB, bw=6361.5KB/s, iops=1590 , runt= 10385msec

tgtd with this patch:
  read : io=65416KB, bw=7295.2KB/s, iops=1823 , runt=  8967msec
  write: io=65656KB, bw=7321.1KB/s, iops=1830 , runt=  8967msec

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>