kvserver: use intent count for gc queue trigger

**Is your feature request related to a problem? Please describe.**

We currently don't trigger GC on the intent count. But in some situations it can happen that a range ends up with lots of abandoned intents but not otherwise a reason to trigger GC.

**Describe the solution you'd like**

Use IntentAvgAge or something like that to trigger GC after a sensible duration. Make sure the cooldown period is active to avoid busy loops.

**Additional context**

X-ref https://github.com/cockroachlabs/support/issues/2236

Jira issue: CRDB-27157