Accept tahoe-lafs as an optional parameter

Fall back to our internal packaging if it is not supplied.

Also, shift towards less use of overlays.  Explicit arguments are better when
possible because they don't induce freaky action at a distance.

Make ShareStat actually copyable

Also add a test that actually serializes it

Add an HTTP API for price calculation

Parse the byte string pass-value into an integer when initializing the server plugin

Consider lease state (expired or not) when the client computes the price of a mutable write

Require the Tahoe-LAFS API authorization token to access the ZKAPAuthorizer HTTP API.

The entire ZKAPAuthorizer client resource hierarchy is now protected by this
token, ensuring any callers at least have local filesystem access to the
Tahoe-LAFS node.

Report "Parsing redeem response failed" better

it's unclear what a good testing strategy for this logging is.  should switch
to eliot soon.

Add more logging for spending decicions

Spend the ZKAPs when doing allocate_buckets

This makes for ugly logs though probably no functional impairment.

Greatly reduce the interval between redemption retries