In unit tests, we would want to delay the merging of some MVCC
versions to test the transient scenarios with multiple versions present.
In many cases this can be done by holding snapshots to all versions.
But sometimes (i.e. during schema upgrades) versions are added and
scheduled for merge immediately, without a window for the test to
grab a snapshot to the new version.
This patch adds a pause() method to mutation_cleaner, which ensures
that no asynchronous/implicit MVCC version merges happen within
the scope of the call.
This functionality will be used by a test added in an upcoming patch.
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