Fixes #7610, #6676

Add the QDRANT_NO_MULTI_MMAP environment variable to disable multi-mmap at runtime. It disables opening multiple memory maps to the same piece of data on disk in all of Qdrant.

This pattern of opening multiple memory maps is broken in some environment, corrupting storage on disk. That's why you can now disable it on demand.

An example of such environment is Docker on Windows with a storage mount to the Windows host. This flag effectively brings us better compatibility.

Usage example:

# run without multiple mmaps for better compatibility
QDRANT_NO_MULTI_MMAP=1 ./qdrant

Once memory maps are first used a warning pops up if this feature is disabled:

# create collection with mmaps
$ bfb -n 100

# Qdrant logs now show:
2025-11-26T10:46:22.885080Z  WARN memory::mmap_ops: QDRANT_NO_MULTI_MMAP is set, you may see reduced performance

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