CREATE FUNCTION: dlopen/dlclose not need to mutex lock handler

`dlopen` returns handler that has reference count internally, we do not need to lock with the `map handler>`, the same for `dlclose`. A simple `map handler>` is cleaner.

Look for the CREATE FUNCTION implementation and the map handling dlopen/dlclose handlers. The issue suggests removing mutex locks around the filename-to-handler map and using a simpler fnname-to-handler map. Start by finding the relevant code in the source tree, likely around function loading or dynamic library management. Understand the current locking mechanism and reference counting to ensure thread safety without the mutex. Verify changes by building and testing the function loading functionality.

索引モデルが issue の本文から書いたものです。