Consensus method requests from cometbft are synchronous, but a portion
of the work of Commit is launched in a goroutine, so we block a
subsequent BeginBlock from starting new changes. We do this by
acquiring a semaphore with max concurrency of 1 at the start of
BeginBlock, and releasing it when the changes from Commit have finished
applying. A mutex is rarely held for longer than the duration of a
local function, while a waitgroup does not provide atomic Wait/Add
semantics that fit here.