Crash in LMDB during a domain reload while a merge operation is happening.

There are no real steps to reproduce this issue, as it is based on timing.

Unity can crash during a domain reload if a merge operation is currently ongoing. This is because we release all native resources, such as the LMDB database, when the OnDisable is called, which happens before threads are stopped.

Steps to reproduce:

1. Open Unity
2. Start an indexing operation.
3. Wait until the background progress says "Merging artifacts...".
4. Trigger a domain reload.

Actual results: 

Unity crashes.

Expected results: 

No crashes.

Reproducible with versions: 6000.3.0b1, 6000.4.0a1

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): Windows 11

Notes:

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