Don't stall syncing if gap is too large

[tomaka]

cc #2793

After trying to analyze the problem, I've decided to leave it to future-me.
It is very complicated to determine which entries in the list of "blocks known by a source" should be kept and which can be thrown away. There are several other TODOs related to that in the code.

For now, this PR fixes the sync stalling.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
        -568 ┊ smoldot::sync::all_forks::sources::AllForksSources<TSrc>::remove_known_block::h675080f2e1d47233
        -238 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h3a1c23e25b4b37db
        -230 ┊ alloc::raw_vec::RawVec<T,A>::grow_amortized::h57c5ad59fe99ddd6
        -181 ┊ alloc::raw_vec::RawVec<T,A>::allocate_in::hdb59a704b8ae2af6
        -131 ┊ alloc::vec::Vec<T,A>::extend_desugared::h7a13f870f6e49acd
        -108 ┊ alloc::raw_vec::RawVec<T,A>::current_memory::hb8ff6a56806efb21
         -95 ┊ <alloc::vec::Vec<T,A> as alloc::vec::spec_extend::SpecExtend<T,I>>::spec_extend::h95bce498991f7a36
         -95 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter::h91341b1ae4e29986
         -78 ┊ <alloc::raw_vec::RawVec<T,A> as core::ops::drop::Drop>::drop::hbac2f1e1893f2d36
         -58 ┊ <<alloc::vec::into_iter::IntoIter<T,A> as core::ops::drop::Drop>::drop::DropGuard<T,A> as core::ops::drop::Drop>::drop::h5d4bbc79c69014d7
         -58 ┊ core::alloc::layout::Layout::array::h10c9b14e0d61b748
         -56 ┊ alloc::raw_vec::RawVec<T,A>::reserve::do_reserve_and_handle::h3ed58e84da2fdbe1
         -55 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::next::hf3329ffcaabaa5e5
         -52 ┊ <alloc::vec::into_iter::IntoIter<T,A> as core::ops::drop::Drop>::drop::h58a11575b847f5d0
         -39 ┊ alloc::vec::Vec<T,A>::reserve::h4e73390be96a90e1
         -16 ┊ smoldot::sync::all_forks::AddBlockVacant<TBl,TRq,TSrc>::insert::h7197847b81be47c2
         -16 ┊ smoldot::sync::all_forks::AnnouncedBlockUnknown<TBl,TRq,TSrc>::insert_and_update_source::h398f43956a0fcf66
       -3691 ┊ Σ [17 Total Rows]