• Move default and override limits, and associated methods, out of the Limiter to new limitRegistry struct, embedded in a new public TransactionBuilder.
• Export Transaction and add corresponding Transaction constructor methods for each limit Name, making Limiter and TransactionBuilder the API for interacting with the ratelimits package.
• Implement batched Spends and Refunds on the Limiter, the new methods accept a slice of Transactions.
• Add new boolean fields check and spend to Transaction to support more complicated cases that can arise in batches:
  1. the InvalidAuthorizations limit is checked at New Order time in a batch with many other limits, but should only be spent when an Authorization is first considered invalid.
  2. the CertificatesPerDomain limit is overridden by CertficatesPerDomainPerAccount, when this is the case, spends of the CertificatesPerDomain limit should be "best-effort" but NOT deny the request if capacity is lacking.
• Modify the existing Spend/Refund methods to support Transaction.check/spend and 0 cost Transactions.
• Make bucketId private and add a constructor for each bucket key format supported by ratelimits.
• Move domainsForRateLimiting() from the ra.go to ratelimits. This avoids a circular import issue in ra.go.

Part of #5545

Ideally ratelimits/bucket.go should be renamed ratelimits/transaction.go but that will make the diff harder to review. We can do the rename in a follow-up.