Use URL + Public key as cache key in Boulder Publisher

[mcpherrinm]

Summary:
Publisher caches logs based on pubkey only, not pubkey and URL.

boulder/publisher/publisher.go

Line 71 in 2678e68

c.logs[b64PK] = log

Steps to reproduce:

1. Submit a certificate for SCTs
2. Change URL of a log, and reload boulder-ra with the new configuration.
3. Submit another certificate

Expected result:
Publisher should succeed at submitting to the new URL

Actual result:
Publisher uses a cached log client with the wrong URL

Additional details:
In our non-production environments, we use Boulder's ct-test-srv. If we redeploy ct-test-srv without restarting the Publisher, submissions fail. We could work around this by giving more static URLs to our test logs, but it would be nicer if Boulder handled this.