Add thinktime_blocks_type parameter

Add thinktime_blocks_type parameter

This patch adds a new parameter thinktime_blocks_type to control the
behavior of thinktime_blocks. It can be either `complete` or `issue`. If
it is `complete` (default), fio triggers thinktime when thinktime_blocks
number of blocks are **completed**. If it is `issue`, fio triggers
thinktime when thinktime_blocks number of blocks are **issued**

Tests:
jobfile1:
```
[global]
thread
kb_base=1000
direct=1
size=1GiB
group_reporting
io_size=96KiB
ioengine=libaio
iodepth=8
bs=4096
filename=/dev/qblkdev
rw=randwrite

[fio_randwrite]
thinktime=2s
thinktime_blocks=4
```

jobfile2:
```
[global]
thread
kb_base=1000
direct=1
size=1GiB
group_reporting
io_size=96KiB
ioengine=libaio
iodepth=8
bs=4096
filename=/dev/qblkdev
rw=randwrite

[fio_randwrite]
thinktime=2s
thinktime_blocks=4
thinktime_blocks_type=issue
```

Results:
Current HEAD:
fio jobfile1:
	write: IOPS=5, BW=24.6kB/s (24.0KiB/s)(98.3kB/4002msec); 0 zone resets
blktrace:
	11 reqs -- 2s -- 8 reqs -- 2s -- 5 reqs -- end

This patch:
fio jobfile1:
	write: IOPS=5, BW=24.6kB/s (24.0KiB/s)(98.3kB/4001msec); 0 zone resets
blktrace:
	11 reqs -- 2s -- 8 reqs -- 2s -- 5 reqs -- end

fio jobfile2:
	write: IOPS=1, BW=8190B/s (8190B/s)(98.3kB/12002msec); 0 zone resets
blktrace:
	4 reqs -- 2s -- 4 reqs ... -- 4 reqs -- 2s -- end

Server:
fio --server=192.168.1.172,8765
Client (On the same machine):
fio --client=192.168.1.172,8765 jobfile1
	write: IOPS=5, BW=24.6kB/s (24.0KiB/s)(98.3kB/4001msec); 0 zone resets
blktrace:
	11 reqs -- 2s -- 8 reqs -- 2s -- 5 reqs -- end

fio --client=192.168.1.172,8765 jobfile2
	write: IOPS=1, BW=8191B/s (8191B/s)(98.3kB/12001msec); 0 zone resets
blktrace:
	4 reqs -- 2s -- 4 reqs ... -- 4 reqs -- 2s -- end

Signed-off-by: Hongwei Qin <glqinhongwei@gmail.com>
[axboe: fold patch 3 into this one]
Signed-off-by: Jens Axboe <axboe@kernel.dk>