Added a backfill in 7-day windows, and fetches now keep their data on a failure

Backfilling a long window (an early --since, or a long time offline)
hit the API's rate limit, which could still be exceeded after waiting
once, and then everything fetched so far was lost.

Before each Find a Tender source's worker starts, a start point more
than 7 days ago is now backfilled in 7-day windows bounded on both
sides, oldest first, each saved before the next is fetched. A window
that fails is saved as far as it got and fetched again from its start
after the source's interval. The worker then carries on from the end
of the last window.

A 429 is now waited out up to sources.<source>.rate_limit_retries
times (default 5) rather than once; fts-once --ignore-rate-limit uses
0. FetchAll returns what it fetched before an error along with it, and
the workers save it, so a failed run no longer loses its data.
findatender's FetchAll and FetchOnce take RequestOptions, which gains
Until (updatedTo), and RateLimitRetries replaces AllowRateLimit.

internal/config/README.md

@@ -72,7 +72,9 @@ supported.
 | `sources.<source>.enabled` | bool | `false` | For every source |
 | `sources.<source>.interval` | duration | `10m` | For every source; e.g. `30s`, `1h`. Must be greater than 0; turn a source off with `enabled` |
 | `sources.find_a_tender.api_root` | string | `https://www.find-tender.service.gov.uk/api/1.0` | URL of the Find a Tender API. Empty: the source fails to start if enabled, and `fetcher fts-once` fails |
+| `sources.find_a_tender.rate_limit_retries` | int | `5` | How many times a request refused for the API's rate limit is waited out (for its `Retry-After`) and retried. 0 fails at the first; can't be negative. `fetcher fts-once --ignore-rate-limit` uses 0 |
 | `sources.fts_mock.api_root` | string | | URL of the mock FTS server (see `internal/mock`). Empty: the source fails to start if enabled |
+| `sources.fts_mock.rate_limit_retries` | int | `5` | As for `find_a_tender` |
 | `sources.sleeper.block_shutdown` | bool | `false` | Test source: if true, its sleep ignores shutdown |
 
 The current sources are `find_a_tender`, `fts_mock` and `sleeper`. See