Is your feature request related to a problem? Please describe.
The memory consumption of webp-server may grow to >500MB when processing 5 concurrent requests that require webp conversion. Such workload causes a great burden on conventional virtual servers (with 1GB ram) and may trigger the OOM killer.

Describe the solution you'd like
Implement a limit on the number of concurrent webp conversion tasks.

Describe alternatives you've considered
The memory usage problem can be partially mitigated with the following options:

1. Limit the concurrent request numbers in HTTP servers.
   This is a common strategy to prevent the backend from overloading. However, it's ill-suited for webp-server, as conversion requests will block subsequent requests that can be directly fulfilled by returning the original image file.

2. Convert every image file to webp in advance with -prefetch
   However, users still need to restart webp-server to perform pre-conversion every time they add some image files, or some concurrent requests might still cause excessive memory usage.

Both methods are not optimal enough for real-world usage, hence I think this problem should be addressed in webp-server itself.