Currently, _shardsvrBeginMigrationBlockingOperation issues a _configsvrSetAllowMigrations command without an lsid, but SERVER-121209 will require an lsid for every _configsvrSetAllowMigrations call. Since _configsvrSetAllowMigrations is invoked on the user's command thread (call site → coordinator), once we start attaching an lsid the tassert 10090100 will be hit because the opCtx sessionId won't match the command lsid.

Therefore, we need to start issuing _configsvrSetAllowMigrations from a thread other than the user's command thread.