[9.20] fix: usr: Fix assertion failure in the glue cache
requested to merge backport-4691-fix-data-race-between-free_gluetable-and-addglue-9.20 into bind-9.20
Fix an assertion failure that could happen as a result of data race between free_gluetable() and addglue() on the same headers.
Closes #4691 (closed)
Backport of MR !9126 (merged)