seek_accurate, memory usage

Hi,

_seek_accurate()_ consumes a huge amount of memory. Calling _seek_accurate()_ on a 30 min 800MB video ends up taking more than 28GB in memory. Is there a way to reduce the memory consumption?

OS used: CentOS 7

Code:
```
import resource
import time
import decord as de

vr = de.VideoReader("video.mp4")

vr.seek_accurate(30)

time.sleep(10)
print(f"resource {resource.getrusage(resource.RUSAGE_SELF).ru_maxrss / 1000000} GB")
time.sleep(10)
print(f"resource {resource.getrusage(resource.RUSAGE_SELF).ru_maxrss / 1000000} GB")
time.sleep(10)
print(f"resource {resource.getrusage(resource.RUSAGE_SELF).ru_maxrss / 1000000} GB”)
```

Output:
```
resource 10.801124 GB
resource 21.972212 GB
resource 28.476392 GB
```