* Make the methods of `CThreadInterrupt` virtual and store a pointer to
it in `CConnman`, thus making it possible to override with a mocked
instance.
* Initialize `CConnman::m_interrupt_net` from the constructor, making it
possible for callers to supply mocked version.
* Introduce `FuzzedThreadInterrupt` and `ConsumeThreadInterrupt()` and
use them in `src/test/fuzz/connman.cpp` and `src/test/fuzz/i2p.cpp`.
This improves the CPU utilization of the `connman` fuzz test.
As a nice side effect, the `std::shared_ptr` used for
`CConnman::m_interrupt_net` resolves the possible lifetime issues with
it (see the removed comment for that variable).
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