import: avoid over-spliting schema objects

[Related ticket ](https://github.com/cockroachlabs/support/issues/2167)(internal)
[Related convo](https://cockroachlabs.slack.com/archives/C2C5FKPPB/p1679509768216649) (internal)

We should avoid over-splitting schema objects, especially for small datasets measured in storage bytes, even if the splits are short-lived. It’s a stability risk — these short-lived ranges are all non-quiescent so consume CPU like we saw above. For a cluster provisioned with some finite # of vCPUs, it’s very easy then to destabilize itself, vCPUs that would’ve been sufficient in the steady-state post-import+1h. Once https://github.com/cockroachdb/cockroach/pull/98820 lands early 23.2, this “oversplitting” will show up more prominently and cause outages.

Jira issue: CRDB-25799

gz#16454