Enable proper rate limiting functionality

Use the --qps option to control per-hostname rate limits. The limit is
currently global and applied independently to each hostname, before
DNS resolution (due to the current structure of the crawler that
performs DNS resolution in the worker), but it should be definitely
better than nothing.
1 job from better-queue in 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #5725
run_tests

00:00:17

 
Name Stage Failure
failed
run_tests Test
=== RUN   TestSchemeScope
--- PASS: TestSchemeScope (0.00s)
=== RUN TestURLPrefixScope
--- PASS: TestURLPrefixScope (0.00s)
PASS
ok git.autistici.org/ale/crawl 0.353s
? git.autistici.org/ale/crawl/analysis [no test files]
FAIL git.autistici.org/ale/crawl/cmd/crawl [build failed]
ERROR: Job failed: exit code 1