So with the switch to a new linter, we get a lot of new warnings,
and the majority of them are harmless probably, but a few might be
real. Variously just use _ to suppress warnings, or report errors.
There's probably things here that deserve better fixes, but we can
always revisit it.
This commit adds the ability to remove an 'available shard'
from the shard cache. This does not affect shards known to be
available because of local data.
This commit removes the previous `MaxShard` tracking and replaces
it with an `Available Shards` set tracking. This allows sparse shard
tracking without implicitly tracking all shards in between.
