cleanup_indexed_database_transactions() is called on every event
loop spin. It was calling associated_connections() which allocates
a GC::HeapVector every time, creating massive GC pressure on
JS-heavy sites (217K allocations observed on x.com).
Switch the hot path to use a GC::RootVector instead, which lives
on the stack and avoids GC heap allocation while still being
visible to the garbage collector.
Rename the existing methods to make the return type explicit:
- associated_connections_as_heap_vector() for callers that
capture the result in GC::Function lambdas
- associated_connections_as_root_vector() for callers that
just iterate safely within a single scope
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