allocation_strategy: Fix standard_migrator initialization
allocation_strategy: Fix standard_migrator initialization

This is the continuation of 30722b8c8e, so let me re-cite Rafael:

    The constructors of these global variables can allocate memory. Since
    the variables are thread_local, they are initialized at first use.

    There is nothing we can do if these allocations fail, so use
    disable_failure_guard.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>
Message-Id: <20201028140553.21709-1-c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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