fix : Accepting a RefreshProposal should clear all pending unsigned RefreshProposals

[1xstj]

Summary of changes Changes introduced in this pull request:

• Resolves #724

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d5512ea) 18.79% compared to head (4cec5cf) 18.47%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #725 +/- ## ========================================== - Coverage 18.79% 18.47% -0.31% ========================================== Files 80 80 Lines 5958 5969 +11 ========================================== - Hits 1119 1102 -17 - Misses 4839 4867 +28 ``` | [Files](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/725?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools) | Coverage Δ | | |---|---|---| | [pallets/dkg-proposal-handler/src/lib.rs](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/725?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools#diff-cGFsbGV0cy9ka2ctcHJvcG9zYWwtaGFuZGxlci9zcmMvbGliLnJz) | `48.82% <ø> (-7.08%)` | :arrow_down: | | [pallets/dkg-proposal-handler/src/impls.rs](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/725?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools#diff-cGFsbGV0cy9ka2ctcHJvcG9zYWwtaGFuZGxlci9zcmMvaW1wbHMucnM=) | `75.41% <90.91%> (-0.59%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/webb-tools/dkg-substrate/pull/725/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webb-tools)

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