Tune backfill runtime defaults

[dutifulbob]

Summary

• increase the default backfill batch budget and shorten the pause between backfill passes
• raise the default repo lease and fetch interval to better match large-repo production timings
• make the git index timeout configurable and raise its default

Validation

• go test ./...
• go vet ./...
• go build ./cmd/ghreplica && go build ./cmd/ghr
• npm run docs:check

[dutifulbob]

Validation:

• go test ./...
• go vet ./...
• go build ./cmd/ghreplica && go build ./cmd/ghr
• npm run docs:check
• gh pr checks 12 --repo dutifuldev/ghreplica

Change summary:

• raised the default backfill batch budget to 25 PRs / 3 minutes
• shortened the default pause between backfill passes to 5 seconds
• raised the default repo lease to 15 minutes and the default repo min fetch interval to 1 minute
• made git index timeout configurable and raised its default to 5 minutes