Add the keygrabber sample, sink contract, retry queue and InfluxDB sink

[mikelangmayr]

First slice of the keygrabber: the storage boundary only. No daemon, no polling and no config yet, so nothing here runs on its own.

• Add libby/keygrabber/ with Sample (keyword, group, peer, value, units, timestamp) and a Sink protocol of connect / is_connected / write / close. Nothing in Sample is Influx-shaped, so a second backend is a new sink rather than a change to the collector
• write returns how many samples it stored, which can be fewer than it was handed, and raises SinkWriteError when a batch failed and is worth retrying
• Add RetryingWriter, which wraps any sink and holds failed batches in a bounded queue with exponential backoff, dropping oldest when full so a database that stays down cannot grow memory without limit. It starts no thread and never sleeps: the caller drives flush_due() and the clock is injectable, so the backoff tests run in a millisecond
• Fresh batches queue behind an existing backlog rather than overtaking it, and a retry that fails again stops the drain instead of hammering the backend
• Add InfluxSink for InfluxDB 2.x: one measurement per keyword name, tags group / peer / units, single value field, whole batch as one request
• Skip null values rather than raising. lasterror is nullable on every daemon and None most of the time, so a collection reading a whole peer would otherwise fail on every tick. Confirmed live against a daemon: a % read returns lasterror as {"ok": true, "value": null}
• Write ints as floats, so one peer reporting 0 and another 0.5 for the same keyword cannot collide as int against float and have the write rejected. bool is checked first, being a subclass of int
• Give absent units a none placeholder, since Influx drops an empty tag value and a keyword would otherwise split across two series depending on whether it declared units
• Keep the Influx token out of InfluxConfig.__repr__, so a traceback or a logged config cannot leak it
• Add an influxdb extra and install it in the tox test env, so the sink's tests run in CI instead of skipping. They skip cleanly when it is absent
• Add libby.keygrabber.sink to the API reference; the Influx module is left out because it imports its client at module scope and the docs build does not install the extra
• 28 sink tests, pylint 10.00/10, and the -W docs build verified locally. The write path is covered against an injected write API and the point mapping against the real client's line-protocol serializer; no live InfluxDB server was exercised

[prkrtg]

LGTM!