on_request/on_response stored callbacks in append-only Vecs with no way
to remove one, so a crawler that installed a response-body capture hook
for one phase could never detach it: it fired for the shared client's
whole lifetime, re-registering across navigations piled up duplicates,
and (because the client is shared per context) it saw sibling pages'
requests too.
Tag each callback with a stable id. on_request/on_response now return
the id and off_request/off_response (remove_on_request/remove_on_response
on the client) detach a specific one. Storage is Vec<(u64, Callback)>;
all fire sites destructure the tuple. Closes#408.
SGavrl
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