[memo] assume self-join stats cardinality continuity

[max-hoffman]

Self-join stats estimation is particularly expensive because all of the buckets exactly overlap. If the index is unique, the cardinality distribution will not change. If the index is non-unique, the cardinality will expand proportional to rowCount/distinctCount.

before
BenchmarkOltpJoinScan-12    	    1766	    694524 ns/op	  462834 B/op	    8240 allocs/op
after
BenchmarkOltpJoinScan-12    	    2460	    481166 ns/op	  193569 B/op	    7129 allocs/op

sysbench perf here: dolthub/dolt#8159

[nicktobey]

The new file sql/query_props.go seems unusued and not connected to the rest of the PR.

Otherwise, this looks good.

[max-hoffman]

Whoops thanks for flagging, didn't mean for that to sneak by me.