Introduce a new SpinningSynchronousClose API that allows to synchronously close
a connection that executed asynchronous statements, by spinning the events loop.
This is expected to be used rarely in particular cases like database corruption.
It is currently [noscript] since the only consumer is cpp, in the future we can
evaluate removing that, if we find more uses for it.
MozReview-Commit-ID: 7SPqutoF9jJ
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extra : rebase_source : d053e308192f335dfdca089668e56115b068ff8c