With regards to closing the looked-up querier if an exception is thrown. In particular, this requires closing the querier if a semaphore mismatch is detected. Move the table lookup above the line where the querier is looked up, to avoid having to handle the exception from it. As a consequence of closing the querier on the error path, the lookup lambda has to be made a coroutine. This is sad, but this is executed once per page, so its cost should be insignificant when spread over an
entire page worth of work.
Also add a unit test checking that the mismatch is detected in the first place and that readers are closed.
Fixes: #13784Closes#13790
* github.com:scylladb/scylladb:
test/boost/database_test: add unit test for semaphore mismatch on range scans
partition_slice_builder: add set_specific_ranges()
multishard_mutation_query: make reader_context::lookup_readers() exception safe
multishard_mutation_query: lookup_readers(): make inner lambda a coroutine
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