renter: update directory heap to prioritize explored directories, add logic to reset the heap

7 jobs for <span class="ref-name">3491-add-from-dirheap</span> in 49 minutes and 37 seconds (queued for 1 second)
Status Job ID Name Coverage
  Lint
passed #199135168
lint

00:04:15

 
  Test
passed #199201570
integration-tests

00:21:53

75.0%
passed #199135169
legacy-tests

00:07:03

48.2%
passed #199227622
package-tests

00:16:26

87.0%
failed #199135171
integration-tests

00:25:00

75.0%
failed #199201558
package-tests

00:16:49

87.0%
failed #199135170
package-tests

00:16:16

87.0%