Fix race condition in snowflake broker

There is a race condition with the snowflake heap that has been causing the broker to crash several times a day. This race condition has existed in the broker for several years, but some recent updates as well as the host migration managed to shake it loose.

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This race condition is causing the snowflake broker to crash repeatedly and often since the migration. We noticed because CollecTor stopped collecting metrics since the restart on 14 November 2019.