Accelerate Base64 Encoding

[ATATC]

Base64 encoding has become the bottleneck of the performance of LEADS since video-related features are added into LEADS. Accelerating this process will benefit not only video capture but also AI features.

[ATATC]

Although in #251 we decided to use multithreading to get rid of the poor performance of Base64 encoding, there's still a little room to improve.