Problem: mempool iteration is not thread safe

[yihuang]

Closes: #675

Solution:

• hold the lock during iteration, blocking concurrent modifications

A different approach than #681

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[mmsqe]

SenderNonceMempool seems still have issue

[yihuang]

SenderNonceMempool seems still have issue

what issue?

[mmsqe]

SenderNonceMempool seems still have issue

what issue?

no now, error from cb is handled