Resource temporarily unavailable when calling VideoReader or VideoLoader multiple times

Hi Joshua,

I tried to call VideoReader() or VideoLoader() various times but it crashed in the middle. To reproduce the error, I provide the sample code below:

```python
import decord
for i in range(250):
print(i)
#a = decord.VideoLoader(['/mnt/SSD/Kinetics_trimmed_videos_train_merge/7E60em35UUw.mp4'], shape=(8,340,256,3), interval=8, skip=0,shuffle=3)
a = decord.VideoReader('/mnt/SSD/Kinetics_trimmed_videos_train_merge/7E60em35UUw.mp4')
# a.reset()
```

The program shuts down when i is 230 or 231 with an error message saying:

```bash
Traceback (most recent call last):
File "test_code.py", line 6, in
a = decord.VideoReader('/mnt/SSD/Kinetics_trimmed_videos_train_merge/7E60em35UUw.mp4')
File "/home/yzhao/.local/lib/python3.6/site-packages/decord-0.0.1-py3.6.egg/decord/video_reader.py", line 21, in __init__
uri, ctx.device_type, ctx.device_id, width, height)
File "/home/yzhao/.local/lib/python3.6/site-packages/decord-0.0.1-py3.6.egg/decord/_ffi/_ctypes/function.py", line 175, in __call__
ctypes.byref(ret_val), ctypes.byref(ret_tcode)))
File "/home/yzhao/.local/lib/python3.6/site-packages/decord-0.0.1-py3.6.egg/decord/_ffi/base.py", line 62, in check_call
raise DECORDError(py_str(_LIB.DECORDGetLastError()))
decord._ffi.base.DECORDError: Resource temporarily unavailable
```
For VideoLoader, the error message is similar:
```bash
Traceback (most recent call last):
File "test_code.py", line 4, in
a = decord.VideoLoader(['/mnt/SSD/Kinetics_trimmed_videos_train_merge/7E60em35UUw.mp4'], shape=(8,340,256,3), interval=8, skip=0,shuffle=3)
File "/home/yzhao/.local/lib/python3.6/site-packages/decord-0.0.1-py3.6.egg/decord/video_loader.py", line 24, in __init__
uri, shape[0], shape[1], shape[2], shape[3], interval, skip, shuffle, prefetch)
File "/home/yzhao/.local/lib/python3.6/site-packages/decord-0.0.1-py3.6.egg/decord/_ffi/_ctypes/function.py", line 175, in __call__
ctypes.byref(ret_val), ctypes.byref(ret_tcode)))
File "/home/yzhao/.local/lib/python3.6/site-packages/decord-0.0.1-py3.6.egg/decord/_ffi/base.py", line 62, in check_call
raise DECORDError(py_str(_LIB.DECORDGetLastError()))
decord._ffi.base.DECORDError: Resource temporarily unavailable
```

The environment that I use is also attached for reference
+ Ubuntu 18.04
+ ffmpeg 3.4.6-0ubuntu0.18.04.1

Will appreciate it if you could look into this. Thanks!