This commit removes the use of a purge height from the nursery
store, instead tracking the last graduated height. This serves
to indicate the last height that the nursery store successfully
graduated. It also adds a method for retrieving the set of all active
heights below a particular threshold, e.g. the last graduated height,
allowing the utxo nursery to replay these heights on startup for the
specific purpose of re-registering for confirmations of outputs in the
height index.
Also includes:
* improved error handling when pruning channels
* more aggressively signals errors when enumerating height outputs
* removes use of variadic functions in Incubate, AwardDiplomas
* renames AwardDiplomas to GraduateKinder
* short circuits channel maturity test after finding 1 non-grad output
* replaces prefixed outputs in height buckets with files, instead of
empty buckets
This commit introduces a new type called a NurseryStore
which abstract many of the persistent operations required
by the utxo nursery. The code in this commit is
intentionally introduced as dead code, as the utxo
nursery will later be modified to hook into the new
database. It is designed to support incubation of
commitment and htlc outputs, and includes full
chain segmentation!
