make repsert a special case of repsertMany

[njohnwalker]

repsertMany admits backends to provide a custom implementation via connRepsertManySql. in the case of the postrgres backend, repsertMany becomes an atomic operation. It would be nice to take advantage of this for repsert as well. (it's fairly well known as a gotcha in our codebase but this would be a nice QoL improvement for us)

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[parsonsmatt]

awesome, thanks!

[parsonsmatt]

Uploaded as persistent-2.14.4.3

[njohnwalker]

no, thank you!