update throttler latency query

[morgo]

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Fixes #286

This updates the throttler latency query based on a previous misunderstanding:

• We can use the MAX().. of all the applier threads as long as we use APPLYING_TRANSACTION_IMMEDIATE_COMMIT_TIMESTAMP
• The workers not applying will return NULL for this column, so the MAX() value only considers active workers.

This can now technically return a NULL value, but it should not provided replication is enabled (which is checked in pre-flight). For simplicity though, we also wrap the final output in an IFNULL.