Data listener reads are implemented as flat_mutation_readers, which
take a reference to the listener and then execute asynchronously.
The listener can be removed between the time when the reference is
taken and actual execution, resulting in a dangling pointer
dereference.
Fix by using a weak_ptr to avoid writing to a destroyed object. Note that writes
don't need protection because they execute atomically.
Fixes#4661.
Tests: unit (dev)
e03c7003f1