Use concurrent hash map to avoid potential infinite stall

[strangesource]

Description

An infinite stall was reported when playing back DRM content with preprocessHttpRequest network config configured.
preprocessHttpRequest is an API that is being called on multiple threads and hence must be treated in a thread safe way. However, the map used for the in-flight preprocessing is NOT thread safe and hence could cause infinite blocking preprocessing calls.

Changes

• Use concurrent hash map to avoid infinite stalls
• Add error log in case a completer can not be found as this should never happen

Checklist

• 🗒 CHANGELOG entry