Extract the per-link lock-out and a JSON-file record store (second host: heed) #3

Two hosts now carry the same code: thorwhalen/annals share_api and i2mint/heed#90 heed/review_access.py.

1. Per-link lock-out: two limiters (per client per link; per link from everyone, locking for a window) plus a locked dict. Candidate: caplinks.AttemptGuard(rate_limiter, link_failure_limiter, lock_s, clock) with allow(link, client) -> bool and failed(link). Reviewer notes: prune the dict, and note it is per process even with a shared limiter.
2. JSON-file record store (MutableMapping: atomic write via mkstemp so 0600, checked keys, tolerant listing): annals' _ShareRecords and heed's _JsonRecords are the same job.

Relates to i2mint/heed#88.