serialize DuckDBClient.sql

[mbostock]

Ref #1469. This uses a WeakMap keyed by the string literal strings, effectively creating a queue per SQL cell and per sql tagged template literal. I’d prefer to implement a more general solution that also helps out with e.g. concurrent fetch (maybe in the Observable Runtime?), but this is an easy fix for probably the most common instance of this problem. The performance of the mag slider on the SQL page is dramatically improved under fast interaction.